Really depends where it is and even with lifeguards it can be difficult
Two contrasts about how people behave:
weston super mare. mudflats. clear signs about not going too far out as there is the danger of getting stuck. These are regularly ignored by those who don't know the territory and people have to be rescued by lifeboat all the time especially in summer as it goes really fast.
Burnham on sea. Very, very dangerous part of the beach where young children and older ones get swept off with unexpected wind and then sucked under and very difficult to rescue. There is a lifeguard with loudhailer who you hear every two minutes telling people to leave a dangerous area. There have been a number of fatalities.
I would guess in most places due to funding cuts there may not be adequate provision of lifeguards etc.
If it is your back yard and you know the beach and conditons well. Maybe.
As for looking after two with the younger one as well. I absolutely and totally wouldn't. What happens if the younger one runs off suddnely in the opposite direction to the water. You can't just leave them. I'ts not doable.
I do feel some parents and some men in particular are a bit nonchalant about this. They shouldn't be in my view.
How about a half way house? There are an increasing number of lidos and safer enclosed outdoor swimming areas...
The difficulty is perhaps that the other parent views you as a 'stick in the mud'. Difficult to change that if that is the case. I think once you have experienced or heard of a difficult situation you are more cautious.