It sounds to me like a water moccasin, but I may be wrong. Try calling animal control and see if they will come and get it out.
Where do you live? Assuming you are in the northern US, it sounds like a harmless black snake or water snake. He may not be able to get out on his own, you might want to use your long-handled skimmer to lift him out. (Northern) water snakes are ill-tempered, and will bite if provoked, but are not poisonous.
If you are in the southern US, or another country, you should look online. In the southern US, there are poisonous black-colored snakes that swim. If you are unsure, don't mess with it until you get someone to check it.
Post a photo, if you can, and people can be more specific.
Add your voice to the conversation. Click here to answer this question.