You'll be fiiiiiiiiiiiiiiiiiiiiiine.
Trust me. If you do happen to fall out (which is
highly unlikely), the adrenaline rush alone will help you fling yourself back in the boat. And give your friends (and the guide....ahem...) a little more credit. They will reach down, grab you by the back of your jacket and basically fling you back in the boat. And I suspect your not going to go completely limp, either. Right? You will be helping the cause.
If you are concerned about your weight preventing you from being lifted back in,
don't. If you fall out (again, HIGHLY unlikely), one of two things will happen.
1. You get hoisted back in. Not a problem.
2. You swim to the river bank.
Just remember, point your feet downstream (not your head, please
)
So, you know how at the beginning of every flight you have to listen to that little spiel about the oxygen mask and putting your head between your legs? That's kind of what this is (and what they will do before your trip). In the highly highly highly unlikely event...
Just remember to pay attention, lean inwards instead of outwards and if you aren't comfortable with paddling, don't. And I'm begging you. Don't let this worrying about being too big to get back in the boat thing ruin your trip. In the end, it will be a waste of good vacation time. "Unscheduled adventure swims" aren't as bad as they sound. And again, they are extremely uncommon.
This does remind me of my first river trip. I was 9 or 10 and the guide was giving us the old "what to do if...." talk on the bus and he told us that if we fell out to swim to the right. On the left sat train tracks and no room for much else (it's against a canyon wall). He said that it was a long walk back to town and if a train came along we would have to get back in the river. It scared the mess out of me (even though it was completely false)! To this day, whenever I take a trip down that river I always make a mental note to "swim to the right". And then I giggle at the utter stupidity of it.
edit: Please tell me you are going to be wearing a jacket?