I'm 17 and I'm a lifeguard. It pays pretty well, there's also potential to be a swim teacher which is quite an easy qualification once you are lifeguard trained. The only thing is that the course is expensive (mine was around £200) but if you secure a job first they will pay for it, at least all the places I have heard of, or they take it out of your wage a little bit each month until you have paid it back. One thing to consider is that you have to do training once a month and renew your qualification every 2 years but you should get paid for training. However, it is quite hard and you have to be a good swimmer.
Other than that, supermarket jobs are usually okay. Sainsbury's tends to have good policies but I think Lidl is the highest paying supermarket at the moment.
Hope that helps a little bit.