IMHO, the ~30$ pointer should be enough for most things you'd like to do underwater. I don't see a reason why paying ~180$ for a green laser. I have two red laser pointers (20$ each, a year ago from LP)- very simple design no switch- only head rotation and it works fine, although during daytime it is sometimes difficult to see the spot.
I don't know what you're going to do with it, myself I don't like pointing a laser neither at humans nor fish. I use it for aligning my strobes together and they are more than enough for this purpose.