Sailors operating on vessels such as the PBR and Swift Boat looked like regular Army infantrymen (obviously without the field gear such as rucksacks, webbing, bandoliers and such). Most boat crews wore standard jungle fatigues, M69 flak vests and M1 helmets without the Mitchell camouflage covers.
Most Nam era infantry figures would work, provided they are not festooned in heavy combat gear. I would have a look at some of Bravo6's resin figures. There are quite a few in 'relaxed' poses that would definitely work.