Getting married underwater sounds fun and romantic until you dive a little deeper and realize what a commitment it is to really do the thing!
I guess there are ways and ways to 'do' an underwater wedding. If you're really into scuba diving then you go that route, which might be a bit extreme for some and for which you also have to have a certification.
There are some beautiful and no doubt memorable wedding pictures taken of just married couples floating at the bottom of fancy hotel pools taken by adventurous wedding photographers. It seems a little gimmicky but like it could be a bunch of fun.
Ultimately, the best bet might be to have a little bit of everything, instead.