I remember growing up, the beaches at Ft W were open and life guard patrolled. They had such a clean white sand beach. They had a roping system to prevent swimmers from going out to far. They also had a floating barge out there.
Its been closed for several years probably since the 1980s. Thus, they stopped taking AS good care of the beach, way down it has grass growing up on the sand and the bottom of the lake is not longer scraped or whatever they use to do to keep it crystal clear.
I have seen once or twice a small gator at FT W. (watch out pet loops, they were seen on the canal banks where you walk your dogs and doggies are a gator fav meal and easy pray. However, I think Dis had done an excellent job keeping them out of there, havent seen one in YEARS.
Back to the lake. Yeah, you can go down and sun and I do see mainly kids wading/swimming upclose. Definately not to far out. They do have signs posted.
I kind of feel its partially because in the Florida hot heat bacterias can breed in the lakes and they can infect you. I know of a kid who went swimming in a (non-Disney) lake in the hot summer and ended up with a serious kidney infection from it.
So between the potential liabilty and probably cost cutting of life guards thats probably why they 'closed' the beach for swimming. Plus, theres lots more boats now than were there when they opened.
(I been to Ft W 1000Xs since it opened when I was a small kid)