Your men's swim trunks should stay above the knee. Any further and it inhibits much-needed mobility. This applies to board shorts too. Some of us prefer to show off a little bit more thigh and some of us like to keep the sun at bay. A good suggestion is to give your knees 3-4 inches of sunshine. Measure from your hip (where you'll tie off the trunks) to your knees to see how long your swimsuit needs to be to give you that extra movement when you're paddle-boarding, taking a breather, or diving in the pool. And don't forget to put on sunblock!
Your swim trunks should first fit around your waist. Not too high. That is, not covering your belly button. They should fit at the middle of your hip bone. After all, they're really just water-friendly shorts. Next you should make sure they fit comfortably around your thighs. If they're too tight you'll be readjusting all day at the beach. Think about it: you wouldn't wear briefs to the beach but you want something that has the relaxed benefit of boxers.
You might notice that jammers are the go-to swimwear for those water-heads who are racing or competing. But if you're not jumping into a swimming pool with Michael Phelps anytime soon then you can hop into the pool with a classic pair of board shorts. Look for features like chlorine resistance.
Do you think we're going to stop you from doing exactly what we're going to do? Men's swim trunks are great for doubling as shorts. After you dry off and throw a shirt on, you're a trusty pair of boat shoes away from heading off to that barbeque.
The main function is for air flow. When you're out of the water, it's easier for you and your swim trunks to dry out, plus it keeps everything in place. It serves the same function that underwear does but avoids all the air bubbles and discomfort.
If you jump into a lake or pool with non-swimming shorts, you'll notice your pockets filling up like airbags: there is a lot more air-bubble action because the cloth is designed without water in mind. For swim trunks there are holes in the pockets to make it easier for you to swim. After all, you're a man, not a buoy. The holes also let any extra water flow away without the need to invert your pockets.