so if it is not enough does it mean that when this comes out it will need a case just like the gt-31?
I would use a case, yes.
I have seen ziplock in bathcaps for triathletes. GPS does not work well in water.
Slightly off topic:
For phones I have seen rubberbags that would be usable for a watch too. But it would be hard to strap a watch with those.
For windsurfing and similar I guess you want it on the arm, maybe on the sail.
A rubberbag and a phone arm holder would maybe work. I have not seen an arm holder for the wrist, only upper arm though.
The GB-580 would fit better, or the draft design you linked too.
If the device came with (or was optionally sold with) an arm holder where the device could be used with rubber bags, would be a winner for me too, also for running.