I don't think you want to put on a Gumby suit as a precautionary step, for exactly the reasons you mention. They pretty much disable you from doing anything. I think you want to put them on when the next step is into the water.
It's also worth a few words about how canister rafts work, because I think it's often misunderstood. I know I didn't understand it for a long time.
Both canister and valise rafts alike have a painter line that is (or should be) tied to the boat. When you manually release the hold down strap on a canister, it's doesn't inflate. When you shove it overboard, it doesn't necessarily inflate either. It inflates when you yank on the painter. Now as part of the flight overboard the painter might get tugged, but it's the tug that inflates it, not release from the cradle, and not contact with the water.
Similarly, when the canister hydrostatic release is activated, it just cuts the hold down strap, just as if you release it yourself. And similarly, the raft doesn't inflate. The canister just floats to the surface. The release activates at a certain depth, so it will only activate as the boat sinks. It's a last resort to set it free in the event you were unable to shove it overboard on your own.
Now you have a sinking boat and a floating canister with the painter tied to the sinking boat. When that line draws tight, it deploys the raft just as if you yanked on the painter yourself. The painter is designed to break before the boat can drag the raft underwater, and the raft is now free.
I think one misconception is that the hydrostatic release inflates the raft, and leads people to envision the raft becoming entangled in rigging. It's just the canister trying to float free, so much less likely (but not impossible) to get entangled.
