Very nice start so far. I like the shape of your cave-lake.

I agree with Xorne about the grid placement. Some people drop a grid down over their entire map, but I like to put mine only over anything that is low to the ground and can actually be walked on. Everything else goes above the grid, if possible. Also, things like fallen logs, large rocks/boulders, etc., can be walked/climbed on, but I usually put them above the grid as well, just because I think it looks better that way.

For water, what I've found I like to do is make the grid look slightly different in colour and much more subtler than the land/ground grid, usually by changing the layer mode and lowering the opacity, just enough that it's still visible for gameplay (if the characters are wanting to go swimming/wading or whatever) but not as strong/noticeable as the land/ground grid.

Are your water depth colours on separate layers? If so, try blurring them (I use Gaussian Blur) so they blend together with each other more naturally, rather than having a hard line between each one. You can also play with the layer opacity if you want to tweak them lighter/darker. What I do is paint my water depth layers with a black round brush (over my base water texture), change the layer mode to Luminance Legacy, apply a Gaussian Blur, and then start playing with the opacity of each one until I have a natural-looking, seamless transition between them all.

One other suggestion: maybe expand the area around your lake just a little bit (on the left, right, and top sides), so that there's not only room for the characters to walk around the perimeter of the lake, but also some extra room to add trees, bushes, logs, rocks, etc. Doesn't have to be a lot...just enough for a natural "frame" of sorts. Unless the lake is supposed to be in an empty, grassy area...in which case, nevermind! It's just that I did something similar with one of my maze maps, and thought that type of thing would like nice around your lake as well.