Hi,
I regularly kayak and dive as well. I have an animas vibe pump which is waterproof but before that I had an old d-tron pump which isn't waterproof. I used a small dry bag that I did up as tight as possible on a long tube then stuffed it down my sports bra. I wore a wet suit thermals and a decent dry cag. I found that that worked well enough, I not very good (even now 7 years later) so would regularly swim my roll is shocking and I usually just chicken out and pull my deck. It seemed to keep my pump dry enough, but at your own risk do it, as you'll have to claim on your insurance or buy a new one if anything happens, I did work quite well for me though. Dry suits are pricey and are a pain in the bum with all the rocks doing white water stuff they can quite easily tear whenever your swimming. Develop a good role and get a decent cag (my club always has some nicer ones tucked away so the freshers don't see, first come first served) a decent small dry bag will cost around a fiver. In fact if you were up in Aberdeen I'd just give you mine it's in my car boot pile. Good luck with your pump and have fun on the rivers.
Laura