The best beach for swimming may not be the best for other reasons - such as the practicality of clambering down a hundred steps with a pushchair. No, an ideal beach for swimming needs to have water which is the right depth, smooth sand underfoot and a lack of suspicious floating things.
There are lots of lovely Isle of Wight beaches that are just too shallow for a proper swim. I once walked out for a swim at Priory Bay (Seaview) and ended up in Gunwharf Quays Shopping Centre in Portsmouth with my trunks still dry. I've had similar problems at St. Helen's and at various other popular spots around the east Wight (Appley at Ryde and Bembridge can be like this at low tide).
For example, Hanover Point is perhaps my favourite spot in the whole world but there is a huge stretch of rockbed, which has the cheek to remain hidden at high tide. Many visitors happily run into the sea from the golden sands and stub their toe a few seconds later. If you listen carefully you can hear expletives echoing around the bay.
The rocks also have peaks and troughs so even with wetsuit boots on you find yourself up to your neck one minute and then up to your knees the next.
However, we can get some clues from which beaches get Blue Flags, which at the time of writing are Sandown, Colwell, Ventnor and Yaverland. Incidentally, wouldn't it be better if the blue flags were actually pictures of turds with a line through them to indicate the cleanliness of the water? Anyway, I digress.
Whitecliff Bay is another good option as it is nice and sandy, although getting there is difficult as you have to walk through a holiday park and then down a hill so steep that it almost requires crampons.
As I said earlier, Hanover Point is rocky, but if you walk towards the Compton Bay end of the beach it is much sandier and there are frequently big waves. It might not be ideal for training for a swimming competition, and it doesn't ever have lifeguards so you do need to be a bit cautious, but it is unbeatable for some traditional seaside larking.