Cleanliness of the water, both sea water and drinking water does not seem to be a problem.
The sea water on the east side of the island is certainly clean and clear.
I don't think you want to swim on the lee side of the island for a variety of reasons. Not the least of which is that we saw a crocodile sunning himself in the mud on the side of a pond during our first visit four years ago.
In the past, we could drink tap water with no problems. However, the Belican Beer distributor just south of town will deliver beer and softdrinks by the case and drinking water in a 5 gal plastic jug. Just call them or stop by and order. They will deliver to your hotel or condo. Very reasonably priced. Good luck!