Well, ther arte some 'bambo bushes' near the water, if you have a decent bow and a lot of arrows you can use it for cover and snipe it.

Otherwise, at first, stand there anyway to make the hydra close in, when it's near the edge of the water, move toward it. It may take a few tries to find the sweet spot, but there is a piece of shallow water that, when looked a from straight up, kinda looks like a hill, it's right in front of the hydra, you'll recognize it when you see it, as soon as you find the sweet spot, wait, pick a head and sever it, rinse and repeat untill it's dead.

Just run past it. Eventually there will be a very narrow path, and a curved branch pointing down.
Hide behind the branch but just poke out enough so the hydra is able to have a straight line of sight on you.
If it starts swimming back it will swim into a tree and die due to glitched fall damage.
Easy souls and a scale or two.

If you really want to beat the hydra run past the maneater shells then the hydra should jump from the left to the right , at this point you should be close to a tree in front of the tree there is a steep bank that you can use for cover and the hydra heads should then be fodder to hack off. The only thing you shoud watch for is that on occasion the mushroom man parent could emerge from the tree behind you. The best course of action is to get the lord vessel first and then sprint pass the hydra and then you can warp once you reach the dragon and bonfire at the end.

Run right at it. Stop near item on the shore and wait for Hydra to close in.
When it starts to attack with its heads alternate between guarding and attacking the heads.