I was Told ours do. Never got to try it tho.
It makes sense that they would, but I don't think that I'd want to rely on it.
It was a long time ago, but from what I remember we were trained that in case of a planned or unplanned water landing to:
- remove the waist belt.
- unhook the chest strap.
- unhook one side of the reserve and tuck it back.
- jettison the backpack (still captured by the lowering line).
- Stage hands on leg strap buckles.
Upon contact with the water:
- Release leg strap buckles.
- Slip out of the harness.
- Inflate UDT vest.
- If necessary, follow a seam to the edge of the parachute.
- Signal "OK" for pick up.
That picture was before my 13th jump, where I needed to make the decision to conduct a water landing, a tree landing, or run with the wind to try and make the LZ, but that's a story for another time.