I scuba dive and snorkel a lot and in general spend a lot of time in and around sea water. Tough environment for knives - that's why I carry cheap. I don't take my good knives anywhere near the beach. Suncream, salt, sand, sweat = bye bye knife. Ever wonder why diving knives, except the high-end titanium ones are made of cheap stainless and are invariably dull OTB? You don't do underwater surgery down there; you cut monofilament, marine plants, rope, or pry stuff. You need a serrated edge, a hook blade for fishing line and such, and a thick, preferrably blunt tipped blade for prying. A diving knife is more like a knife-shaped tool, not a real knife as we normally know them.
I carry a medium-sized no-name diving knife (the kind explained above) in my BCD, fastened to a D-ring. I can reach it with either hand, and if my buddy needs a knife, he can find it easily. It dangles some, but being small there's no risk of snagging.
The original poster mentioned swimming trunks, so I think he may be talking about something less specific than a scuba diver's knife. Beach utility and SD, perhaps? For that purpose I, too, recommend a small FB. If it must be a folder, you can't go wrong with Spyderco. Anything smaller than Delica is a waste of space, I think. Unless you strut around in Speedos you can hide anything in your swimming trunks. Carry options are innumerable.
One carry option: I used the sleeve of an old wet suit to make a "beach & running wallet" that I carry around my right wrist. It has a 2" wide strap and a small pocket with velcro closure, just big enough for car or home keys and/or some cash, credit card and such. For paper money I have a watertight, zip-locked plastic pouch inside the wallet. I have seen similar contraptions in surfing shops but mine cost about 2 bucks to make. I sometimes clip my Delica on that strap, inside or outside. For retention and ease of deployment I have a brightly coloured, adjustable lanyard that I slip around my wrist. I can draw and deploy the knife using only one hand. The neoprene and yellow nylon cord scream "beach" without hinting at a cruel nasssssty blade. The steel will stain but proper care keeps it from rusting. I'm going to get me that Salt ASAP, sounds like just what the doctor ordered.
If only Spyderco made a diving knife. I'd buy one, or five.