I recently added (from scratch so to speak) a lake for swimming in my game using a tutorial on MTS2. It looks lovely and the stairs, board, and slide all work to my satisfaction. They used it for a few hours, getting in and out to do other things via the ladder as normal, then one of my sims left this way:

He swam past the pool ladder then jumped/crawled out at the front steps and walked into the cottage then dove back into the floor after going in:

Then to my surprise swam to the bathroom:


Since then the two residents seem to arbitrarily decide to walk or swim around certain areas at will. Not in a glitchy or jump bug way, it appears more by choice then anything like that. One time they'll swim from the stove to the dining table, next time they'll walk. They either walk or swim to locations outside or inside, but not all locations (i.e. some they never swim to). It's amusing to watch but of course no one I know swims to dinner let alone to the sofa to watch TV

.
Neighbors, guests, friends nor service npcs do any of this behavior. Everyone else walks normally. Residents are the only ones I've seen use the lake.
The tutorial mentioned there may be a little swimming into fringe areas but said directing them back to the ladder should stop it. However when I send them to the ladder they don't always go directly to it. Sometimes they'll swim around the entire perimeter of the house (not on direct lines and using different paths) before returning to the ladder to get out.
Is there a way to correct this--outside of scrapping the entire lot? I wanted to package the lot for upload but not like this of course.
Here you see large this lot is:





...so swimmiing the long way back to the ladder takes quite some time.
Thanks in advance for any help you have to offer.