Xanda has the right idea: "I live in Hawaii, so what better way to trash the dress, than to swim in it?"
The dress isn't gonna trash itself, you're gonna have to do a little of the work.
My friend can still fit into her wedding dress from 1985. For her 25th anniversary, she celebrated by hiring a photographer to take pictures of how ridiculous the dress is, and how awesome it is to trash it.
You have to go to great lengths to marry the merman of your dreams.