There used to be a Freeview channel, on British TV, that I sometimes watched with interest and some envy. It aired in the early hours of the morning and featured a small number of Thai women, some trans and some, I believe, cis. Of those that I think were trans, a couple were certainly pre-op. All of the women always kept their bikini bottoms on.
On one occasion, a very attractive woman, normally well-tucked, could not keep her bulge, and its obvious shape, hidden. She even tried slapping it down while her companion, who I believe was a cis woman, fell about laughing. I'm ashamed to say that I also found it funny.
However, there was a happy and possibly useful ending. To her credit, the afflicted trans woman was not too mortified to return to work. Subsequently, though, she wore bikini bottoms with a frill on the front. It effectively broke the outline of whatever was contained in her bikini bottom.
I think that other pre-op trans women who wear bikinis might think about doing the same.