Sanya as a city doesn't have much to offer, and this bay is also nothing amazing. The water is a bit dirty.
Saying that, it is still better than the majority of Chinese beaches, even better than the famous Silver beach in Beihai (which is rubbish by the way). There's a nice view, especially around sunset, it's very long, and it's right in the city so you don't need to make a special trip out.
If you can rent a car and explore a bit, Hainan has plenty of nice beaches that you can have all to yourself. Skip this one.