The trouble with taking photos of the male form is what to do with your hands; if you push them towards the camera you look as though you are about to grab the viewer, if you place them in your lap you block the view of your swim briefs, putting them out to the sides or behind your back looks odd.
Idle hands do the devil's work so find something to hold; on a beach you'll find driftwood, old rope, fishing rods, bags, water bottles and towels. Hold the object in a way that makes sense; driftwood and rope can be weapons, fishing rods and bags are tools, if you have plenty of water you can drink some and let it spill down your chin and chest. A towel can be draped over your shoulders and held in your hands or allowed to flow out in the wind, adding colour to a photo. With all these props you add interest for the viewer, creating a backstory in their imagination as to what you were doing on the beach that day in your swim suit.
If you want to be photographed lying sunbathing on your towel then put your hands behind your head and smile up at the camera, you'll look confident relaxed and happy with your body.