Swimming in continuous big volume water like that is not fun (speaking from experience
Best option in such a scenario is to conserve energy (lying on your back -i.e. deffensive swiming - so you can see the next wave and time your breath) and take guidance from someone else in their boat who can tell you where to go - they will be able to see much further and can tell you swim left/right/to the eddy/bunch up as there's a big hole or whatever. If they are confident enough they might let you hold the tail of their boat - but they'd probably want to know you well enough to know you'll let go if they need you too!
In the case your on your own it's a difficult one. Defencive swimming's purpose is to protect you from hitting rocks and other nasties which can hurt you (not much of an issue in big volume) and to give you sight of what's coming up ahead so you can turn on your front and aggressively avoid hazards and get out of the river (and on big volume you won't be able to see past the next wave).
So I'd be aggressively swimming for a bank - It's not an ideal option, you still won't know where your going & you can't be sure there is an eddy there and that's where the nasties are going to be if they are around at all (overhanging trees, shallow rocks etc..), but it's better than staying in the river.
One thing I did find helpful is holding onto your boat - it floats higher than you so it does give you a bit more perspective of whats coming up and allows you to save some energy till the point where you need to aggressively swim to get yourself out of the water!
The important point (as it is with all swims) is getting yourself out of the water - so deffensive swiming is always going to be of limited help as it'll never get you out!