Q: Or at least what can I say happened to it to get a new one :)
A: I wouldn't worry , i'm sure there will be several clauses in your terms & conditions that fully cover you for submerging your delicate electronic device into a large expanse of salty seawater. I mean, surely when they design mobiles, they should realise that everyone does this every day as part of their normal use of their phone. When contacting t-mobile, i wouldn't mention the 'sea' bit, just say it's faulty