Yes. Mine has done this many times.

DS has been wearing his rash guard all day at camp. It is light weight and decreases he need for sun screen on the arms. His choice, we have a tshirt in his backpack if he wants to change

Not only the UV protection, the rash guards, IMO, are less stretchy and are less likely to get air pockets/bubbles under the shirt which are a pain in the water. I've worn both, including a dri-fit shirt when I didn't have a rash guard available. The dri-fit shirt is more stretchy and adds a bit of drag when swimming. The rash guards are less stretchy and a lot more comfortable in the water. If I have a dri-fit shirt, I often just take it off and swim without it and then put it back on when I get out of the water because it will dry fast.

Yes, but rash guard may be more comfortable.

My DS has worn both and prefers the feel of the rash guard when swimming.

Except rash guard shirt has uv protection, can a boy wear dri-fit shirt to the beach or pool or water splash park?