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That sounds like it would be super awkward even without the trans stuff.
Tips
Doubling up on bottoms or wearing swim shorts over a bathing suit should be adequate to conceal things, esp if the inner layer is tight. Swimming/beach gear also tends to include various skirts and things. Tbh in that setting I'd rather not wear something super revealing anyway.
Part of it is that I don't even own any swimwear right now, and I'm definitely not buying any for a work party where I'm only actually out to one coworker. The tips would be what I'd do if I was though so thanks for that, but alas, I'm an idiot who decided not to be out for the 4 months of work I had to do. Ugh.
Ahh yeah, just wear something casual. Treat yourself to a cute beach date next year. 😎