Brief skin tight Lycra swimwear worn in a quiet spot in the garden where you are not overlooked, on a sunny summer's day is sensuous, your body is likely to react with an erection and a spillage of precum. It's a natural human male reaction that should feel pleasurable. If you play with it you will cum but so long as no one is watching that's ok.
If you do cum rinse out your swim briefs as soon as possible to avoid stains.
Many swim briefs especially those intended for swimming training or diving are lined, the lining stains very easily as it soaks up body fluids. If you look around online you will find unlined swim briefs that will show damp spots instantly, black fabric will hide an erection and a damp spot while light coloured fabrics display your bulge.
So go on Ebay and order some brightly coloured unlined swim bikini briefs with a skimpier cut than your current ones and test them out to see if this issue is a pleasure or a problem:
https://www.ebay.co.uk/itm/134736209...Bk9SR-zP2urqZg
If that works out try a swim thong:
https://www.ebay.co.uk/itm/204706521...9SR47Y6e rqZg
If you are going to a beach or lake take enough swim briefs so you don't display damp spots in public, or slip on some shorts if you need to cover up.