When the flight spell falls from the Giant Mantis, it is no longer able to land on a body of water, I receive the echo, "You need a boat to go there." The mantis is, however, in flight above the body of water.

I found that this occurs whenever I have cast flight prior to shifting into the mantis and it falls somewhere along the way. I am, however, able to enliven fly and can then land on the water.

If I shift into a mantis and am not yet flying, I cannot enliven fly.

The mantis will still like the air even after the flight spell fails.