So my Galaxy S went for a bit of a swim on the weekend...
I ripped the battery out once it was recovered and left it to dry for a couple of days but I found that it would boot up automatically as soon as the battery was inserted and constantly reboot or get stuck in a reboot loop of the boot loader. I did a fair bit of googling and found this to be a common problem with people not knowing how to fix it.
I pulled mine apart, by unscrewing all the screws in the back and levering off the back plate with my fingernails and then found that if I unsoldered the little micro switch board for the power and bridged the terminals it would work fine. Then I soldered two wires onto the terminals, popped the power button out of the backing plastic and threaded the wires through the hole where it was. Put it all back together and now I have a ghetto spec galaxy s with two wires sticking out the side. Connecting the wires acts as the power button.
My guess is that the sealed micro switch has water inside it. I'm going to let it dry out and try testing if it still works and if not I might try levering it apart on one side to let it dry.