On St Thomas I like Coki beach over the more crowded Maegens Baty. All facilities, chairs food, bathrooms, scuba, snorkelling and right next door is a paid attraction called Coral World. Second Choice would be Saphire, third choice Maegens. I personally think these beaches are nice enough and would not waste the travel time to St Johns.
On St Martins I am a fan of Dawn Beach. Again rentals and food available , no crowds , pretty beach . There is a drop dead gorgeous pipeline break off of the reef offshore.
Friendly vendors. Good food available from the chair rental guy. He makes a killer rum punch .