I think you will be wanting really narrow sides.
There is a choice between a high cut front and a Japanese style low cut front.
With a high cut front your soft cock can sit upright, with a low cut front it will have to sit diagonally upwards. An erection in a low cut front may well pop out.
You can choose between a lined or unlined front, with thin unlined stretchy fabric the shape of everything will be evident.
The rear can be full, 3/4, 1/2 or thong, the more your reduce the rear the more remote the beach you should chose.
If your suit is low cut the rear won't entirely cover your ass crack leaving some rear cleavage. This can be fun.
Remember that white and yellow can go completely see through when wet especially if unlined.
Have a look at this from Brave Person:
https://www.ebay.co.uk/itm/144786447... 9SR_7Sx660YQ
And this from Desmiit:
https://www.ebay.co.uk/itm/274344138...Bk9SR-b77K-0YQ
Desmiit also do a swim thong:
https://www.ebay.co.uk/itm/233148358...Bk9SR-T77K-0YQ
Seobean are always fun but very low cut:
https://www.ebay.co.uk/itm/194358696...9SR9 LNn7C0YQ