One of these would be somewhere between swimming and using an inflatable raft. Your pack would have to be sealed in a trashbag or similar, and "towed" (presumably what you'd do if swimming). Fishing Float Tube. This is one of many products of its type. Can't find weight, but one reviewer gave points for being "lightweight" and when uninflated/folded, fit in his backpack (which you wouldn't do, but some indication of portability). By their nature, these are sturdy construction.

Typically used with "frogman flippers" for fishing, but for your purposes, maybe a short, lightweight paddle such as this.