i really like the direction that this is going, man.
especially the water. normally that kind of super-low water where you can see the dirt tile underneath it is a turn off, but in your postapocalyptic wasteland i actually think that it's the right move. imo your transition version is great, but you should lose the few floating rocks that aren't connected to the land part. i get what you're doing there and i like it a lot (it will be great if you can keep them but use those tiles sparingly as a few variations)—but when tiled i don't think it will look very good.
also regarding the colors, i REALLY liked that version with the purple. i am a hardcore hueshifter though and that kind of stuff always appeals to me. also the contrast is more pleasant—it has a better sense of depth than the more recent ones that you posted. i'd say GO FOR IT—just if you do, be sure to do the same hueshifting effect on other tiles.
like that cactus for example. the perspective is much better—even though it looks kinda like a bean (it might help you to draw the cactus directly onto the dirt tile, especially if it's one of your first object tiles for the set—that helps me with my rpg perspective on tiles for stuff like that). but i don't like the colors on it—hue shift that motherfucker like you did with the purple above it and it will look really swell.
also when it comes to the shape of the cactus don't be afraid to exaggerate a little bit—i for example would go to town with the spikes
as for the second version of the water—why not use both? they look like they'd both be plausible for different variations in depth.
good luck dude. this is a good thread keep it going