I live in a seaside town and my kids have grown up at the beach and in the sea. I'd still be concerned about an unsupervised group of 15 year olds on an extremely hot day. Their age is absolutely relevant. It's proven that teens have an invincibility complex and take more risks when in groups. Completely different from a sensible adult who can take a decision to seek shade, use sunscreen, stay hydrated, use the sea carefully etc.
All of this is exacerbated by this being teens who may not have a lot of experience in the sea.
Paddleboards and inflatables can be treacherous as it's easy to get pulled away from the coast by the current (even when it looks calm). Our lifeboats are frequently out rescuing people who've been swept out.
Swimming in waves is completely different from swimming in a pool. The sea can be deeper or shallower than you expect when jumping in off a groyne or jetty.
The fact that there's a breeze makes it feel cooler, which makes you less inclined to reapply suncream but doesn't stop you getting burnt.
15 year olds generally have very little sense when it comes to drinking alcohol. Drinking will make them more likely to indulge in risky behaviour, plus more likely to get dehydrated. Friends of mine who live right on the beach frequently need to call an ambulance for poorly teens during the summer.
'She'll learn from experiencing independence' is no use if she dies or becomes seriously unwell or injured in the process.