Never drowned in a crypt, so sorry can't help you there, had to swim under a few humps, but never anything that last any distance.
Some crypts are harder than others, if you have a spawn heap, they are much tougher. Mark it on map and come back later.
The Stagbreaker tip is gold, half clear a blockage, then bang it on the ground. Do not entirely clear a blockage unless you can kill the spawn. If you have elites, walk away until you are geared up.
Dont forget to make a campfire inside at the entrance. That spot is also really useful as a fall back, again ground banging here can kill stuff that can not reach you up the step. Archers not included ofc.