so have you ever played high difficulty at all? the effect of drowning is much less than you seem to think, it mostly just makes the tier 1 dungeons go more smoothly and means you aren't walking around with half a kit in randboss land. even if you get a truly exceptional fixedart/randart it will not make t2 dungeons significantly easier and will be phased out completely by the time you reach dreadfell.
Yes, from my experience playing easy/normal/nightmare/insane. Haven't tried madness. Abusing drowning makes the game about one level easier. So winning on insane with drowning is about the same achievement as winning on nightmare without drowning.
yes drowning definitely makes the one hard part of the game that cant be trivialised by digging (reknor vor:A briagh) easier, where you will be decked out in t3 randarts minimum anyway, and maybe 10% of a level ahead because of drowning
in reality its just that insane is very easy no matter what you do and drowning just makes the early game less tedious to play through.