Hello,
This happened to me three times, when i play mesmer in pve/pvp and die under water i am able to swim up but as soon as i use my skill to leave a downed clone behind I get stuck. I am not able to swim or move anywhere as if i was downed on land. It must have something to do with my clone or maybe the harpoon weapon i always carry with me.
Either way its a bug worthy to report.