Hydras will spawn in light level 15 on a single open fence gate in a void.
They also spawn in water, redstone, places where they collide with blocks, and on glass, bottom slabs, and bedrock.
As far as I can tell there's no way to spawn proof an area against hydras other than making it a complete void.

Oops. I forgot to change line 550 . My apologies. This will be fixed in the next version. For now you can turn down the hydra spawn chance in the config.

I'm using In Control for now to make it some spawning rules.
I'm enjoying the mod, looking forward to an update!

This should be fixed in the latest 2 releases.