Why I can't build a pool on a waterfront beach on The Sims 4?

@Simtasticgallery  Unfortunately, the restriction is exactly as it says: "Cannot build below ground level on waterfront lots"

So no basements and no in-ground pools on a waterfront lot. You could build your pool above ground (make a room for the first “floor” then go up to the second and put a pool in; add stairs and turn the floor by the pool into decking). Or you could build on a non-waterfront lot and use landscaping tools to put a pond in near an in-ground pool.