I'm thinking that a container designed to keep a liquid IN, would be equally as suitable to keep water OUT. ...pressure of depth notwithstanding.
So, a screw-top container (jar, bottle, canister, etc) would be ideal.
I too am planning an underwater cache, and I'm using a diver's box. Obviously this is designed to keep water OUT, under pressure. Maybe it's overkill, but it depends how deep you are putting the cache. Anything over 10 feet or so, and it'll be under enough pressure that an ordinary plastic screw-top canister might get compromised. If you put it just under the surface, it'll be out of site in murky water, or amongst reeds or rocks. (Of course it goes without saying - keep it safe.)
I also like the idea of doubling up - one container inside another, to be sure.
I think your idea is great! How many people would drive or walk past your lake every day, and not realise there's a cache there?