If one is swimming, perhaps a shirt is not necessary. But at other times, most men (and woment) would do better to cover up. A loose cotton shirt (yes, even one with long sleeves) is cool and keeps the sun off the skin.
It is not just a question of modesty (although there is that). Most people simply look better fully clothed. Although there are exceptions, the vast majority of Americans do not have bodies that look good half naked.
One more thing. Although I do not live in Florida, I do live in the desert Southwest, where the weather can get very hot. The people who tell me that they absolutely must wear shorts and tanktops to be cool are the same people who spend almost all of their time indoors in air-conditioned comfort. Just about the only time they are exposed to the heat is when they walk between the front door and their cars.
They look fine. More likely you've bought the Madison Avenue schtick and think that unnaturally skinny and toned people are what's attractive.
I happen to like men with hair on their chests, but one visit to a nudist camp convinced me that the vast majority of humanity look better with their clothes ON.
I am more an Esau girl than a Jacob girl; ever since Sunday School I have looked upon 'smooth men' as deceitful -- smooth man equals smooth talker.