When someone has a deceivingly small bum. On social media it looks like they have a large, rounded bum, instead in real life it is very flat and small.
Any number of poolboys, cabana boys, lifeguards, etc. who work in or around water and whose sexual orientations range from ambiguous to flamboyantly homosexual.
"All these tropical buttfish are deflating my sails."
or
"He's a total tropical buttfish. He hit on Mrs. Windham's son last night at the luau."