This is quite funny because when we were DC on holidays by the sea we either IIRC went in with parents supervising or not at all. Even a stream/river which seemed fairly safe (in 2 different areas), one we just paddled in with relatively little adult supervision, the other seemed slightly more risky so an adult was always with us - I was approx 8/9. Rivers with weirs - we holidayed near one and were told on no uncertain terms not to go in but there was a small pool a 10 minute walk away.
As teenagers (approx 14/15) I recall going to a beach on holiday with friends and meeting exchange students and locals there and we swam but not very far out at all, mostly paddled and messed around! I think a lifeguard was a little way off. A few years later, same story.
I think the best thing you can do is not forbid it outright but maybe say stay between flags/near lifeguards and/or in future maybe send on a safety course.
I was lucky - I was a strong swimmer and we had had 'scare stories' drummed into us so knew how to be safe, I don't honestly think we'd have messed around a lot. Having said that, one close friend of mine, who came with us on holidays and to the beach, I don't actually think she was a great swimmer but we never thought to ask?!
SolitudeatAltitude - you think lifeguards would be aware/alert but not always the case. Some years ago, my DB was with me, an older DC of a family friend and DM - DM was swimming lengths (Olympic sized pool) us kids were swimming normally. Me and DB were/are strong swimmers, my DB was having diving lessons. The DC of family friend (about 2-3 years older than me, approx 16, I was 14, DB was 12) decided to 'drop' us by the shoulders into the water from the edge - yes stupid but a 'game'. Went fine for a while then back DB's head smacked against the edge of the pool on going in and he went underwater and almost drowned. My DM who had finished lengths nearby was just out of the water, luckily dived in, saved him etc... Lifeguard barely aware, did nothing or didn't see..., DM put a complaint into the sports centre afterwards. DB was unconscious (but regained after a day) and had stitches. Luckily DB was ok but naturally scared of the water for years later.
So lifeguards aren't always the most aware/brilliantly trained etc!