We wear snorkeling rash guards every time we snorkel. The number one reason is that we don't have to slather sunscreen on our arms and backs every time we get in the water. Rash guards work better than sunscreen and we never get burned accidentally because we missed a spot. They also provide a little warmth and protection from jellyfish stings.
Snorkeling rash guards are made of Lycra, which is stretchy and comfortable. They come in different weights and thicker ones will be warmer. The better ones have an SPF rating on their fabric, normally about 50. They dry quickly and can be washed in a machine.
Because we primarily wear them for sun protection, we wear long-sleeved snorkeling rash guards. It is also nice if it has a high collar to protect the neck a little too. To the right are a couple of examples of the long sleeved snorkeling rash guards we use.
Instead of the skin tight options above, you can also opt for a loose fit rash guard. The good things about these are that they are a bit more modest, and they are easier to get on and off. But, some people don't like them as much because they have a greater chance of riding up, leading to sunburns on the low back. And if you are in the water a long time the extra fabric can lead to some skin chafing.
In cooler waters, we wear thin (1mm) neoprene wetsuit tops for snorkeling. They keep us warm longer than rash guards and still provide sun protection. And they work for night snorkels too. If you get cold easily, this might be the best of the snorkeling rash guards for you.
Neoprene does not dry as quickly as Lycra and is not as stretchy. Wetsuit tops can be more difficult to get on and off. And they are not washable. To the right are neoprene tops like we use.
Basically this is a full body rash guard, often called a skin suit. We have considered using these so that we use less sunscreen, but have yet to go that route. After all, we do like the feel of the water on our skin. Another problem we have with these is how do you pee? Come on, we all pee in the ocean, don't we? You can get these in Lycra like the ones to the right. Or if you want real warmth you can get a full body thin neoprene wetsuit.
When dry you want the tight versions of these to fit a little tighter than you might think. They are generally a little hard to pull over your head, and then should have no loose material around the body or arms. The reason you want them so tight is because they loosen up when they get wet. Also make sure you don't buy one that is too short. It should easily pull down over your suit bottom when dry so that when you are swimming it does not inch up and allow a burnt back (it has happened to us - you get in the habit of making sure it is pulled down).
For the loose fit style, just don't buy them too big. They should still be snug, so there is not too much extra fabric.
We prefer ours to be primarily white, mainly because when we are out of the water in the sun, they are cooler. You'd sweat like crazy in a dark one.
A cotton t-shirt is not a good idea. First, a typical white cotton t-shirt only has an SPF rating of between about 6 to 8, when dry. And when wet that goes way down. We have seen family members get a good burn through a wet t-shirt.
There are a couple of other reasons not to wear cotton. First, it picks up water-weight, decreasing your buoyancy. Second, since it is not a skin-tight material (particularly when wet), it creates a lot of drag when trying to swim and will make you much more tired. And if you freedive, it is much easier to snag it on something underwater than a skin-tight rash guard.
We have found that if we wait to get to our snorkeling destination, the rash guard that costs $25-40 online, suddenly costs $60 or more at diving and surf shops in touristy areas. So it is best to try to buy your snorkeling rash guards before you go.
Originally these were invented to protect the chest of surfers and boogie boarders when laying down on their rough textured boards when paddling - hence the name rash guard. But they are also worn under wetsuits to provide some protection from all the seams and zippers for the same reason.
In Australia they have taken the rash guard to the extreme. Many companies now have their snorkeling customers wear a full body suit, that covers their head, feet, and hands, called a stinger suit. It prevents sunburn, but is more to keep from getting stung by the Box Jellyfish (that is lethal). We have to say though we would feel pretty dorky in one, pulled on over our swimsuits. And the companies choose some really dreadful colors.