Well I know one hadith of Muhammad(PBUH) which says Not to pray in the sea, because is there where the shaitan iblis put his throne and send his army. (perhaps the shaitan uses water of sea like a portal to his dimension of the world of the jins)
So it is bad to pray inside water, but I saw a diver which does it in a photo, perhaps he is much time there working, and it came the hour to pray and because he is not going to go to surface in much time he prayed there, and muslims pray in their ships if the hour comes.
So by that hadith of the demon,it is bad, and because don't follow the sunna of Muhammad(PBUH), if Muhammad(PBUH) says one thing like bury in earth, put the person on his right side, and all those things, it must be followed and the others things and with more reason, invented in purpose against the sunnah are bad, But if the person dies and other people not muslim make a bad burial to him, like fire, sea, whatever, he is not responsible or guilty of that, because he was death, or he couldn't avoid it before, prepare it etc. so i think it has no humiliation to him. the humiliation is for those who made it in purpose inshallah. and i think it doesn't change in be asked by the angels because your tomb is full of water, i think the soul is in other dimension,
and the body in the sea can be damaged more than in earth, it doesn't matter because Allah s.w.t can resucite it, and the coxis from there is going to grow again the body, the coxis is a part of the body indestructible, and from it can be made all the bodies Allah s.w.t wants.