My Sim likes to swim with the fishes.  Apparently this is what happens when you leave a boat on a community lot and some neat little girl tries to put it away.  Building lots half in the ocean can have comical consequences.
 

They can get out, but they seem to get a little disoriented. You can still tell them what to do.  You can not, however, click on them.  The only interaction you will get if they are completely submerged is ‘Fish,' because the water is what is actually getting clicked on.  I almost decided to leave this lot in my world because it was so funny to watch, but then I decided that wading in the water is nice, but complete submersion is just weird. Also, they can‘t fish on the lot.  She will go to the neighboring lot to fish.  She will do this completely submerged as well, because that lot is in the ocean, too.  However she will not fish on the lot she is actively on when told to.  You think standing underwater looks cool; fishing underwater is a hoot—especially with sharks and alligators overhead.