I'm not sure about specific beaches but there was swimming allowed in the Seven Seas for quite a long time. Nature stayed away when there was a lot of activity on the lagoon, but as nature does, it reclaims its spaces and nature in the form of Gators and Ameba's found the place and locations and Disney slowly closed them down. After the attack and death of that child in 2016 then they got concerned big time. One of the problem was the wealthy geniuses that rented those new, out into the lagoon, cottages were feeding gators and others things so that made more nature show up. Heck, for a while originally there was even a wave machine pointed a the Polynesian. The equipment might even still be there under the water.