One of the EP packs introduced a problem with the water level on certain lots, causing the basements to have standing water on the floor. Although mermaids might like this, most Sims don't!
I found a fix for this while looking at Swimable Lakes tutorials. Simply take the Terrain Tool and dig a small hole in the basement floor. This should force the water level down below the floor level. The basement should remain dry even when the floor is flattened again.
If this doesn't work with your particular combinations of EPs and SPs, try digging a deep hole in the yard then flattening it again.
Reference: Tutorial: Making real lakes swimmable
Note: I had to fix the ground underneath the connection slightly first, because deleting the connection screwed with the elevation of the ground underneath, which screwed with the water level. Remember, if a piece of the bank is pushed down below the water level by any means other than the "Water terrain" tool, the game assumes you want it to remain dry land and adjusts the water level for the entire lot downwards. If you push "lower ground" on a piece of the bank, you can watch your entire lake dwindle and vanish!
P.S. I suspect you could make a "dry river" or "dry lake" in a desert terrain with this trick.