Not UK specific, but having once upon a time worked as a Life Guard at a pool in the United States, I can say that this falls under "Good Samaritan laws" which provide for immunity to lawsuit for bystanders who get involved in an emergency situation. "Good Sams" provide three basic protections:
- If you are a bystander, you have a limited immunity to injuries inflicted in competently administered life saving measures you take.
- If you are an off duty first responder OR not even a trained first responder, you are immune if for whatever reason you do not act to provide life saving measures.
Both of these do have caveats to them, for example, when I was a Life Guard and driving home from work, if I stumbled upon a wreck and provided CPR to a victim, if I did not use my breath mask and the victim contracted a disease from me, the victim could not sue me for medical damages because I was rendering aid that was "Life over Limb"... that is to say, the law assumes that the unconcious victim would prefer me to save his life at the cost of a non-immediate life threatening injury (this is one of the reasons the entire premise of the film "The Incredibles" does not work for me. The injuries sustained by the jumper and the train passengers are not something Mr. Incredible is Civilly Liable, since he was equipped to properly save those people.). And just an FYI, properlly rendered CPR will hurt when the victim wakes up... considering at this point, you are manually assisting them to breath, the greater good demands me to inflict a survivable injury instead of killing them. However, since I was never trained in performing a tracheomtry, if I did that, I could be sued.
Secondly, it does allow me to refuse service if I am off duty for a variety of reasons (My state at least. There are some states that require a bare minimum response, if only to call 911). And while at first this may seem like an asshole thing to do, leaving someone to die, the first thing any first responder would do is assess that they can safely administer their aid. If I'm off duty and see a body floating in the pool and a downed power line in the water, all that I will achieve by jumping into the pool to pull the body out is adding one more body that the next first responder needs to treat. Most people who act in first responder capacities want to help people on the worst day of their lives... but you achieve nothing of you risk your own safety. This is why in mass shootings, you will hear something about ambulances staging... no medic will go in while a nut with a gun is loose in the building... they'll get as close as possible and will roll up as soon as cops give an all clear, but if someone dies while waiting, they can't be liable. This is just as true when off duty.
Finally, and can't stress this enough, in your specific situation, there is a reason why a lifeguard is sitting at a pool (And let me tell you, it is certainly not because I liked the shrill sound of a whistle followed by a shout of "WALK!" directed at a 10 year old for the umpteenth time in an hour while I have a bright read foam tube wrapped around my waist during a heat wave! No I'm not bitter!). It's their job to be alert and attentive and trained to prevent drownings in the water. In fact, when I was doing it, I was instructed by my boss not to allow anyone who just walked up to help with CPR... but I could draft people to assist in other duties such as give a specific instruction to someone to call 911 (always looked for the parent of the kid first, as not only were they best equipped to explain any medical concerns to the operator, but also because the last thing I needed in that moment is a parent in histarics cause his/her baby isn't alert) as well as assisting in pulling the person onto the side of the pool so I could properly render aid (it was a two man job and I was more often than not the only guard on duty... talking a bystander through this part is pretty easy).
Edit: Bit of a nitpick here but Québec and Louisiana are both hybrid civil and common law jurisdictions, and while it gets weird at times, they are both sufficently common law enough that it doesn't matter here. And a quick point of interest, in the Americas, Québec along with Vermont, Misouri, and Rhode Island are the only four districts where you cannot flat out refuse to give aide in an emergency. Out right refusing aid when not on any duty to do so is legal in 46 states and all remaining Provinces in Canada (I don't know how many there are. And barely know their names. I just call them "The Weird one on the East Coast, The French One where they shove all the rude Canadians, The One with Bigfoot, The one that has a Jurassic Era Dinosaur named for it, The snowy one that I'm pretty sure is just a territory, and Toronto).