My worm was near water, i walked away from water. when in secure place i choose the weapon and then my worm drop in water without any reason.
Other times it happened but i supposed a syncronization problem but in this case it's impossible.

Another time... my worm was in the position of the circled stone, i selected mine weapon and then puff... my worm disappears and i heard the "flop in water" sound.

Hey Aelagon, we have also experienced this bug here at Team17. I believe it is fixed now, but if you see it again, let me know. Bob.