Just remember: there are zip lock bags, and then there are ZIP lock bags. The ones with the slide lock could leak ... the single line zip is pretty secure, but they make freezer bags that have 2 zip lines. The bag is a little thicker, but it's much more secure.
Once upon a time, I used to go canoeing ... I found that zip lock bags are your friend! They come in all sizes, and will help keep things dry. I wouldn't want to try diving with it, but for keeping splashes and sudden (shallow water) submersions from damaging electronics, there's nothing better than zip lock bags. Great for sitting pool-side, too. Or getting caught in the rain.
One other thing: if you're just trying to avoid "splash", squeeze the air out of the bag before it's "locked" all the way and it makes it easier to handle. If you're out in water and think you may drop it (or capsize!), LEAVE air in the bag: it will help make it more buoyant and float to the surface so you can find it.
Thanks for posting your idea. It's one of those forehead slapping "Duh! Why didn't *I* think of that?" tips.