Argyroneta aquatica or the diving bell spider lives most of it's life under water. That would be a crazy little spider to keep and maintain but it would sure be different than land or tree dwelling spiders, lives in an air bubble and eats almost anything underwater.
http://www.hlasek.com/argyroneta_aquatica_6913.html
http://www.hlasek.com/argyroneta_aquatica_6913.html