Iain/Ian. Some very interesting observations re: nutrition/fluid over long pieces.
Ian - up to 50k row without any fluids? Wow. Since your PB for 50k in your signature is 3 hours 46 minutes, going nearly 4 hours of sweating without fluid intake seems surprising to me. By way of comparison, some years ago I did a hilly cycling road race in Scotland, and since I was not some pro rider with the benefit of people standing at the road side to hand me fresh bidons, in order to minimise excess weight (given 1L liquid ~ 1kg) I only took one 500ml bottle on my bike. That proved to be a drastic mistake. The race basically took me an almost identical amount of time to your rowing effort, and I remember from about 2.5 hours on I was absolutely parched, and the dehydration left me completely and utterly destroyed by about 3 hours. Had to massively slow down/painful crawl for final number of hilly miles. So any losses (here mainly from additional weight/power required; for erging, due to stroke interruption) were massively offset by gains/limiting the detrimental effect of dehydration on performance. Admittedly that was in summer, but on the other hand it wasn't that hot (since it was Scotland), and there was good air flow, so I'd say overall pretty similar sweat rate to rowing inside with a fan. I would have thought that exercise physiology would suggest that even if you had to almost basically stop rowing, say, 4 x 15 seconds, e.g. lose 60 seconds in strokes, that the offsetting effect on HR drift/fatigue would mean you'd still end up being able to go a fair bit faster overall, especially over nearly 4 hours. But you feel not?
Iain - interesting that you mention few can tolerate gels. I think this perhaps highlights an interesting very practical difference between rowing and cycling: when cycling, even on hard efforts, it is not that difficult, in fact it is relatively easy, to take nutrition on (in the form of gels etc.). Pro riders, as you're probably aware, take on quite large numbers of them, and that is not purely some cynical marketing ploy; it definitely does show in the numbers/performance level. I think your comment corroborates my own personal experience that rowing is much more "digestively challenging", in large part due to both the mechanics of it, plus the greater muscle involvement. A movement that continually compresses/or applies pressure to your stomach/intenstines will by simple logic tend to suggest it is going to cause far more gastrointestinal discomfort if anything is in there. I do know that rowing with any kind of not completed digestion is without one of the worst things, and definitely worse than the equivalent when running or cycling (though they also are unpleasant too).
The consequence of all of this is somehow navigating a trade-off between the Scylla of indigestion and the Charbydis of "bonking"/energy-depletion (or even worse, dehydration). There is a significant volume of literature that shows that ingestion of carbohydrates, IF it can be tolerated - the big IF - makes quite a significant positive effect on performance in endurance activities, even for elite athletes whose liver glycogen/fat metabolism is of course at a peak level.
I think the answer must obviously lie somewhere in the middle. My ~3 hour effort clearly went too far to the intake side, but I find it difficult to believe I'd get a best effort from zero intake (even though that'd mean zero rowing stroke interruption/impairment).
Next time I think I'll go for just two brief stoppages/slowdowns of about ~30 seconds, at the 1 hour and two hour mark, with the smaller gel size (e.g. typical High5 or equivalent) and a more dilute drinks mix. Meanwhile I'll try go go for heavier food ingestion about 5 hours prior, small snack maybe 2 hours prior, and start the row slightly "over-hydrated" by drinking a glog just before I start, the logic being if I can start the sweating process properly before the water has passed through to the bladder, it'll get used. I'd assume the effect of hormones and sweating means that this will work prior to the 20-40 minutes it normally takes for water to fully pass to bladder from bloodstream. That's the theory anyway. I guess if you got it wrong you'd end wanting to stop to pee, so that'd be a nightmare after 20 minutes. Tricky.
Anyway, I don't know what the correct answer regarding intake is, but I am pretty sure it is somewhere between "nothing" and "too much", and that both of those extremes are likely to be bad for > 2 hour efforts
so stopped after 3. HR monitor has been behaving itself but on the blink today & couldn`t get it to work. 
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