That's a SA-MP bug GLENNN, it can't be fixed
I think it can...
AFAIK, mostly the water level is the same throughout SA, so anybody WALKING under that Z coord would most likely be experiencing a bug (or using submarine mods, lol).
Also, you'd have to take the Hunter's Quarry area and make an exception for it (or basically for any other area that goes below sea level)
I think the cheapest solution would reset the position above the water level so the player can swin.
Maybe that all is not really worth doing. After all, it's a MINOR bug.