If a player starts a mission in the water or, on a boat and then subsequently ejects into the water (either manually or via script), the player freezes and can't move other than to ascend or descend.
I tested this with the TRG-20 (TAVOR)... it has also broken one of my missions.
However, if the player is equipped with an SDAR the problem is not there.
Appears to be an animation problem. Strangely, cannot be repro'd with AI as they seem to be able to move freely in spite of this.