Really a grip on the sheets in CC3 takes a little while. I think all you need to do is create a "Lake" sheet that goes below (in the list) your Land sheet.
*To clarify whats happening - your land is being drawn on top o the sea (as it normally should). If you draw a lake on the same sheet as the sea, then the lake will also be drawn over by the land. So you could either cut a whole in the land or (the more easier option) simply create a new sheet for your lake that is drawn on top of the land.
Hope that helps...but if it doesn't, try posting a screen shot so we can see exactly whats happening.