Again Mike don't worry too much about what I've written. That's one person's opinion, there'll be others who will tell you something completely different. I suppose if you do drown there's a chance you may have to resit the whole thing but I'm sure I've heard stories of people in the past who were fished out after drowning and administered the defibrillator (successfully). It depends what mood they're in and how badly they need people I suppose.
TASS is short for a time assignable duty. It's what you're allocated when you're on a blind line and they haven't been able to reach cap with your other rostered duties. It works like a contactable in easyJet, for those who don't know what that is you will be contacted by current ops by 18:00 local at the latest the day before with your duty. If you haven't heard from them put your feet up and have a beer the next day. The only problem is that they prefer to use Time Assignable people before reserve period holders for uncovered work in advance, so it is likely you'll get used. But you're only contactable the day before, not on the day. Hope that's explained it.