First Person Controller and terrain constraints

My terrain is essentially finished, I might add some trees and grass later on but at the moment it is the shape I want it to be, apart from one small problem: I can't currently prevent my first person controller from going into the lake and not being able to get out. I have used step offset and slope limit to prevent him from going up the hills, but unfortunately I'm not sure how this works in the opposite direction. I've tried smoothing out the terrain by raising the parts of the lake that I've lowered, and it may just end up being a tedious process of doing that, but I'm sure there is also a scripting method I can use so I have to find that out.

This is what you can see from the lake. But you can't really do anything!