The guests enter and exit the swim shack to the left of the stairs. It looks like that area is blocked.
Also, if the guests are too far away from the shack when their swim time is up, they revert to clothed and start trying to find another way out of the water. If fences block the entire perimeter, etc., they will drown.
Ive never tried that set up, interesting idea.
The path dead ending at the swim shack has never affected its use in any of my zoos.
The only things that caused my guests to not use the swim shack were not having a clear space of water on either side of the exit (in the back where they come out to swim) and having the swim shack itself in backwards, neither of which seem to be your case here, so I have no idea why this set up would cause your guests to ignore the swim shack.
One thing I do suggest is put a restaurant on the northwest side of the swim shack entrance because swimming and other water activities make guests hungry.