It uses the terrain height to quickly determine whether you're over water or not. So terrain height of <= 0 would cause it to appear.

Not sure if it's a bug / quirk / other ....

On my last flight glide-safe appeared several times when I wasn't over water .... not so much as a garden pond, and at least 4 miles from the coast.  It behaved correctly when I was over water, but had additional spurious appearances. 

Not a major problem, just mentioning it.

Edit:  Thinking about it, I may have been above terrain that is below sea level (Lincolnshire) at the time if that's relevant.

Android - latest version of everything.