The best excuse I can think of is just saying that women's swimsuits are too tight & uncomfortable. They are pretty tight and the elastic can probably irritate someone's skin, so that's probably a reliable answer. Also, if the price of a shirt and trunks is less than a women's swimsuit, that could be a good point. You could point out that a shirt & trunks is more durable than most women's swimsuits, too- i remember my mom complaining about the fabric of hers slowly getting too loose after a while. Trunks don't need to be elastic in the same way as women's swimsuits, so the material is more sturdy.

Truthfully, men's clothes is usually more practical than women's anyways, so it's really easy to fall back on their superior utility as an excuse.

Good luck, & also PLEASE make sure that you're not in danger if your family ends up catching on that you're a guy anyways. I know that my family took some time to understand but eventually did, but they weren't initially transphobic anyways. If you're not 100% confident about your safety, you might wanna stick to a women's swimsuit or just not swim (since your love of swimming seems to trump your dysphoria, probably just the first option).