For first problem: simply model the nurbs plane so it doesn't intersect you land mass. You can either add more isoparms and bring some vertices down or make a hole in it with projecting curves and "deleting" the inside portion.

Second problem: I'm not that good with fluid effects to begin with. You might have to show a picture of what your fluid effect looks like for me to better understand what to do unless someone else comes along that knows. Is it possible to just add another ocean nurbs plane for the lake and tone the waves down a bit? That might work.