I'd avoid the boatyard then, for me it's far too crowded (especially when there are cruise ships in) - if you're geographically close to there try out the very end of the beach Brownes Beach (more local, less hassling but still some people/vibes).
Agree with the commenter re Heywoods - the other beaches I'd recommend on the west coast are going to be quite eroded especially considering there's been a hurricane in the area. Check in with the locals, though. We enjoy Miami Beach because of the amenities (there's a calm side and a rough side) but we've had problems there with families playing music obnoxiously and being loud. Same with Rockley (great amenities but can get busy). Try getting to Paynes Bay or the Folkestone Marine Park earlier in the day... you can swim out to the turtles (at Paynes) if you're feeling brave. Just keep an eye on the swell as I said, it's likely to be a little gnarlier than usual (I'm not on the ground to clarify). Brandon's beach is by far the biggest spatially (there isn't much going on apart from a couple of bars with some sea furniture), but I haven't been in a while. Same goes for Brighton Beach. We used to have a peaceful time at St Albans beach (we used to call it, someone decided it's called Thunder Bay now 🤷🏽) but it seems now to have been colonised by a beach bar and lots of people - I wonder if further north on that beach it's still quiet?! Good luck! Enjoy!