so i have had this problem as well when diving as deep as 5.5ft. What it is, is that the pressure is great enough to penatrate those tiny speaker holes. Also in turn those holes are small enough to retain the water well after getting out. If it is damaging anything it is the life span of the speaker, but I doubt it as I am sure the speaker falls under that ipx blablabla.
what i do is give it as quick and violent a blow as I can when i get out of the pool and it is usually normal right away if not very soon. Also have found that if you get out and lay around for a while to dry off, take the watch off and lay it next to you on the concrete in the direct sunlight (not to long as you don't want that overheat message). This has also proved to sometimes fix it as soon as I take it out of the sun (10-15 minutes maybe?).
hope it helps, don't freak out though.