I suppose calling it a hobby is slightly misleading - it's actually a driving goal, and more of an obsession.
Scuba Diving. In particular wreck diving, usually in the 40-50m range using decompression diving skills.
I try to get in the water at least once every 2 weeks minimum - if I don't I start to get really antsy.
I also play Xbox360, read, enjoy (love) cinema and DVD's and of course animals - my 2 cats mainly. oh yeah, and reading. Lots and lots of reading.