General rule , for all , is maybe some small towels for apt/room use but bring yer own beach towels. This is such a small "price to pay" as "Marina Family" plus super food and service is excellent. Be careful though, as if you enjoy the food and company so much you will never get on the beach.
But.............with 31 deg here , for Arillas Today , at 7pm, I would think the beach is still being used.
Cheers
Negg