In May I did a trip where we swam the packs several times. I carried a down quilt. A trash compactor bag rolled carefully and compressed inside the pack makes an excellent dry bag.

The technique is knowing what to leave outside the dry bag. Rain gear and shelter are no brainers, but the tough things for me are maps, my GORP bag and toliet paper. I can keep my sleeping bag dry, thank you, but I do have problems with other things.