Thanks for your response, STS! I'm turning the big 30 this year; I've worn speedos around my partner (he quite likes it!) as well as at the public pool when lap swimming, but I feel like lap swimming has always given me somewhat of an "excuse" to be wearing them since they're more practical from a fitness perspective. I'm still working on getting up the courage to wear them for leisure.
Regarding my partner, I've actually been trying to get him to buy one in preparation for the trip, but he's much less comfortable than I am (especially around his parents) wearing minimal clothing, so he'll be donning a loose fitting short/trunk style.
I'm glad to hear I'm not the only one dealing with random arousal, and I appreciate the vote of confidence! Assuming I don't chicken out, I'll be sure to share pictures.
Still, I do like the idea of having a square cut on back-up, as a good middle ground!