I would say that this solely depends on what type of things you want to do. Under water sounds, of course you need hydrophone or slightly waterproof contact mic. That'll work out for many things, but at the same time, under water sounds captured with piezzo are slightly limited range. It is what it is, unless you start to incorporate other things.
Seen people doing things like recording stones, metal pieces, and such.. Even Grunt already years ago, in albums like Petturien Rooli for example, some sounds would be generated with wheelbarrow filled with water and stones, mic'ed and you would get odd, almost unrecognizeable sounds you quite can't get what exactly it is, what type of efx or gadgets... but there was none really. Just the sound of "objects".
On Selected Killing 3"CD, all the acoustic sounds were conceptual recordings. When song deal with bloated corpses decaying on lake, it was indeed recordings of lake. Capturing sounds of water and objects in the water. Or the one track with crime scene cleansing, where it almost sounds as if it was merzbow-esque filter sweeps and white-noise-fuzz, but it really is just high pressure water cleaning stone surfaces.
For recordings, dynamic microphone, condenser microphone, shotgut field recording condenser etc.. all been used depending what type of sound is needed to be and how exactly water is meant to sound.
You got some Finnish projects, I would name KSNK and Sick Seed who clearly used some sources like rain. But not like newage'ish relaxing rain, but field recordings of some specific location where rain hits the metal sheet, or some sort of object and all resonance and such is as vivid as the "polyrhythmic" hammering of the rain itself. You can recognize what it is, but it doesn't make it less interesting.
