Probably just any cube would work, as long as you dry out the cube afterwords (don't leave any water inside the cube). I'm assuming that cubes turn underwater like they do above water, so if you want a good cube, get a zhanchi or a guhong.
Just pick a cube that turns well without lube and don't mind getting wet. Some people used type c's or QJ, if I wanted to do some solves underwater I would use my rubik's brand.
^^ ya but the AV has holds where the water can gO in compared to the guhong.
Plus u want a cube that doesn't lock up as much because you are under stress of lack of oxygen
Probably won't matter much, since they're all beginners. The only thing I can think of that would make a difference is sticker quality, and I don't think either of them have REALLY bad stickers. Just get whichever one's cheaper.
I have a lanlan, and find it pretty good. The lanlan, (you can probably find it at about $5) it's the best option if you do not want to spend a lot of money.
The WitTwo is supposed to be very good though if you are willing to spend more.