[Omnirizon]

I don't know how you'd possibly have enough water to allow aquatic creatures to get around on land without killing everyone there.

Just a little bit of water goes along way to killing things. And just a little bit of water ain't near enough to let aquatic creatures survive.

I don't know why reverse thetis makes any less sense then normal thetis. The only reason we can't breath water is because our bodies use more oxygen than gills can possibly draw in through water. So thetis must magically enhance lungs to allow them to draw in massive amounts of oxygen from water. Now you face the problem of the fact that water probably doesn't hold enough oxygen for that to even happen. So Thetis has to make the water magically hold more oxygen. Now oxygenated water has different bouyancy, and nothing could float or swim in it. So thetis magically makes water retain its same bouyancy despite all the extra oxygen. Let's not get into the matter of magically creating oxygen and how that relates to conservation of mass and energy.

Given all that magical finagling. I think a reverse thetis is quite imaginable. It would actually take far fewer broken laws of nature to accomplish. In fact, all you need to do is magically keep gills from every drying out, and they would pull oxygen out of air just fine. You also need to give the fishy people legs.

EDIT: this also gets into the fact why no underwater species with higher thought functions than an octopus ever evolved, despite that fact that life has been present in water far longer than on dry land. Also why mammalian aquatic creatures, like dolphins, are the most intelligent; since they actually breath air. Basically, the amount of oxygen in water is not enough to support higher brain functions. Thinking, it appears, requires a lot of air.