Just returned from a vacation during which I used these both in the water and out and they performed well.  I ordered only a half size smaller and they were fine, but I don't like tight shoes.  A whole size smaller would have been too tight for walking to the beach, but maybe ok for just being in the water.

An added point is that I snorkeled in these without using fins.  They provide a little bouyancy.  You would still need fins for speed and to cover large areas.

I just washed them in the machine and they came through that fine too.