The umpire's role is to ensure a fair race. To do this means to meddle as little as possible, but to be quick to spot problems before they develop so that the race remains fair. Essentially, the start must be fair and the crews must not interfere with each other for the duration of the course.
One major difference I have noticed between the umpiring in the US and in the UK is the tolerance British umpires have for blade clashes. These are not infrequent on the eastern side of the pond. Yet they are easy to spot from long before they happen, and an alert umpire should warn the crew at fault to steer back into its lane before a clash becomes an issue. In one extreme case, in a race I was involved in as an undergrad, the cox of the crew in the center lane of a six-lane course was weaving so badly that the umpire decided to widen that lane mid-race and pushed the rest of the crews over slightly towards their respective directions. Since the weave was forseeable, this was a far more satisfactory decision than waiting for that crew - which also happened to be in the lead the whole race - to interfere even mildly with any other crew. By making this decision, the umpire actually produced a fairer race than one in which he left the crews alone and waited for a clash.
Unbuoyed courses are indeed difficult to umpire due to all the variables. They are made all the more difficult when the course itself is on a river where stream matters. The Tideway in London defies all logic when it comes to steering, and a cox needs to relearn all rules of how to steer before taking on that river. The quickest course is not the straightest, it is the one which maximizes the stream. This means knowing exactly where the stream is, and there is some disagreement. If the umpire's idea conflicts with the coxswains' whose concepts of where the stream is further conflict with each other, it could be a messy race. Even if everyone does agree, the use by the coxswains of gamesmanship during the race could be the determining factor if one cox is able to force the other out of the best water.
Most rivers are not like the Tideway, however. But rivers in England do tend to have stream as a factor, particularly in the Fall, Winter, and early Spring. Other than on the infamous racecourse in Augusta, Georgia, I can not think of a single race I did before moving to England where stream made any difference. Nevertheless, to keep crews honest in a race and heading down the course in their own lanes, most course had at least rudimentary buoys. It is difficult for a cox to see far enough ahead to find a point, so marking lanes along the course helps keep races fair. This does not have to mean buoy lines all the way down the course, but can instead be a single buoy between the lanes at each 500. My home course in college had no buoys, but didn't need to because there was a large bridge at exactly 1000 meters. It was easy enough to aim for the arch assigned to each lane. After the bridge, coxes could re-fix their points on fluorescent orange markers on the shore ahead. But some courses without such easy points did have buoys, and these were helpful.
The 1996 Oxford-Cambridge men's lightweight boatrace ended in a disqualification due to incompetent steering by Cambridge's cox. The women's lightweight race that same day was also marred and called back due to poor steering by both coxes. These were very unsatisfactory outcomes, but most years see some sort of controversial steering at the Henley Boat Races. As a result, I suggested after 1996 that buoys be put up along the course to mark the center of the stream, especially a difficult locations (such as Fawley, where the river bank moves in towards the left lane of the course). This would make it quite clear where the stream was, and would keep coxswains from playing games. If a cox still could not steer, then he would still face disqualification, but at least the decision would be clearer. The race committee apparently adopted this suggestion, but suddenly changed its mind shortly before this year's races (I guess with me out of the country they don't have to pay me any attention any more).
After umpiring races here in March, I can attest to the single most annoying thing umpires face: incompetent coxes on the start. We used floating starts here, which is doable but necessitates that coxswains follow instructions getting their crews alligned. In one case, after spending half an hour trying and failing to get crews alligned in one race, I sent those crews to the back of the line and moved on with the other races, but I was amazed how many coxswains freeze up and forget the difference between port and starboard, or rowing and backing, or tapping it up and holding water, or getting pointed down the course and being blown across the course. In some cases, they could not even figure out which lane they were in, even when I kept telling them. Of course, this is nothing new - I've observed it all before (the Wolfson women's second eight from 1995 will remember LMH's useless coxswain at the start of Thames Ditton Regatta that year and the exasperation of the umpires). Getting onto stakeboats provides another variable as what is essentially a simple operation becomes complicated by stammering coxswains. Umpires can generally be helpful at the start, but it is up to the cox to pay attention to instructions and to respond. One of the coxes' most important duties on race day is to get the crew calmly onto the start - exactly the point when the crew is most tense is when the cox needs to be most in control.
By paying attention on the start and by steering straight, coxswains can make the umpire's job virtually superfluous. An umpire is only there to serve as a judge in case things go wrong. And usually, when they go wrong, the fault is on the cox.
The worst decision an umpire can make is disqualification. No umpire wants to have to disqualify anyone. In one of our races this year, the crew in the lead moved out of its lane across the other two, completely ignoring my warnings. Although the crew in the middle lane had to bounce around in the lead crew's puddles, it was far enough behind so that this had little impact on the outcome. But the other crew did get cut off and have to break stride for a stroke. At a formal regatta, this would have been a clear case for disqualification. But I did not want to resort to that, especially as the lead crew would likely have won anyway, as this happened only 150 meters from the end of our shortened course (conditions on our home Henley-distance race course have not cooperated on either of the days we had home regattas this year, so we had to move the racing to a more sheltered sub-1000-meter course). I pulled in to discuss the situation with the coaches of the two crews, and it was agreed that the result would either be allowed to stand or the race would be re-rowed, at the decision of the members of the impeded crew. That crew elected not to rerow. Although frustrating, this was a far more satisfactory for me than having to disqualify a crew at a match race, when the whole purpose of the race was to get the crews racing and not to determine any championship. Despite what some people think, umpires are not there simply to mess up races with disqualifications - they are there to find fair ways of resolving problems should problems arise. Inept coxswains are usually the problem, not umpires.
But did the umpire at this year's Oxford-Cambridge heavyweight men's race allow Cambridge's cox to get away with too much? Again, I have not seen the race. But, from what I understand, Cambridge's cox figured out how to take advantage of the bizarre currents of the Tideway to the greatest extent possible without crossing the line that would force the umpire to disqualify him. Thus it would not have been Cambridge's poor coxing but Oxford's which is at question here. But races on the Tideway are extreme scenarios. Most races are on rather more normal bodies of water, and most courses are straight, so there is no gain to be had from pushing into another crew's water unless the object is for the weaker crew to disrupt the better one's rhythm through clashing blades. Where there is some question of lanes, then an odd buoy can often answer it.
The umpire should serve as a last resort. That's why he's there - and it is always better if there is no occasion to utilize him.