From what I understand of wire insulation codes, ANY TW or THWN wire (or other wire with a W in the type code) is waterproof, period. Conduits are assumed to be full of water if they run outside, so wet location wire is supposed to work fine in water.
I'm wiring a submersible well pump. 12-2 w/g "well wire" is thus far so much more costly than 3 rolls of 12-gauge wire and I "twist it myself" (the product I'm comparing is just 3 wires twisted, no jacket) that I'm rather tempted to do exactly that, since I can't see that there's anything "special" about "submersible well pump wire" other than its "special bend-over price." At least, if I understand wire insulation codes correctly. There's no "special insulation code just for wells" - the stuff is typically TW (60C - no problem in a hole in the ground that stays near 10-15C most of the time.)