Rash vests
Occasionally, you'll hear people referring to 'rash vests' or 'rash guards'. These are relatively new, but they are a good thing, and you may want to look at them if you spend a lot of time in the sun or like snorkelling off boats.
Rash vests were developed in Australia for surfers, who wanted to wear something to stop themselves getting a rash when they levered themselves onto their surfboards. It was too hot to wear a wetsuit, so they developed colourful spandex and polyester tops that did the job. As it was Australia, and they know something about sunburn, the new shirts blocked a lot of the sun's UV rays - which made it ideal for children. That explains why a lot of beach shots in Australia show kids clothed top to toe in colourful material - it's a defence against the sun.
Other sports took up rash vests too. Snorkellers wear them when they know they're going to be in the sun or on a boat, because they are good at blocking solar radiation and they dry quickly. They're lighter and easier than a wetsuit, especially when it's hot. They may offer a small protection against jellyfish, especially if they're long sleeved. Steve has a rash vest, but he wears his because he thinks it makes him look macho.
So they're worth looking at. A newer development is the 'swim shirt' that is looser fitting, but does the same job.
You may have heard of 'snorkelling jackets", which are high-visibility tops with some flotation ability. The theory is that you can wear them while snorkelling in order to conserve energy. These may be useful, although we haven't tried them. We'd find them too restricting.
They would certainly be helpful for children or beginners. If you want to find out more, check out the Snorkelling Vest Information Guide. No, really.
http://www.snorkelingvest.net
Snorkeling Online is really helpful too. https://www.snorkelingonline.com/pages/how-to-select-the-right-snorkel-vest
There's a whole new industry coping with kids learning to swim. This will need research. You could start with https://www.lifejacketadvisor.com/learning-with-confidence-the-best-childrens-swim-vests
and http://www.independent.co.uk/extras/indybest/outdoor-activity/best-swimming-aids-for-babies-4-year-olds-toddlers-uk-a7125331.html