Quick answer: A men's short-sleeve swim top from Speedo, offered in sizes from Small through 2XL. It gives you coverage in the pool, at the beach and on the boat, without the bulk of a wetsuit.
Plenty of us would rather swim, snorkel or hang out on deck in a shirt than without one, and that should never be the reason you skip time in the water. A dedicated swim top fits closer than a cotton tee, dries faster and doesn't balloon or drag when you swim. Speedo makes this one up to 2XL. If 2XL runs tight in your chest or shoulders, our extended-size collection includes rash guards and tops in larger sizes.
Swimwear sizing varies by brand, so compare your chest measurement to Speedo's size chart rather than going by your usual T-shirt size. Planning to wear it under a wetsuit? Read How to Prevent Wetsuit Chafing for Good. More options: Extended-Size Wetsuits, Drysuits & Dive Apparel.