See my section on snakes at
http://www.fishpondinfo.com/pmamm.htmDepending on where you live, you may have water snakes. They do eat fish. Here in MD where I live, we just have garter snakes and sometimes other snakes go for a swim. While they can eat fish, they are normally not that adept at it.
I have worked in my pond with a snake in it. Just move slowly. I have pulled plants out of my overgrown waterfall only to discover a snake hidden there. I practically touched one a few times but have never been bitten. Snakes hide well so the snake could have been there for a while. The only way to deter snakes is to have a sterile pond with no hiding places or rocks which I'm sure you don't want. You can try to catch and move the snake but it's not easy. If it's small, a net on a pole or a long branch might work depending on how fast he moves. Larger snakes can be removed with snake grabber tongs but I looked into buying a set, and they're expensive. If the snake is a water snake, it will probably stay around. If it's a garter snake, ribbon snake, black rat snake, etc., it will probably leave after a few days. Whether or not the snake eats your fish, tadpoles, frogs, etc., depends on how big they are and how big the snake is (and how hungry).
Snakes are a part of a pond ecosystem and no more cause for alarm than a large bullfrog that could eat your fish. The raccoons and herons that raid ponds do much more damage.