Jumeirah's beaches are sandy, not the local yellow sand, but fine white sand imported from Egypt.
What is the best beach in Jumeirah? The whole coastal zone of this area is one big best beach in Dubai. You can swim wherever you like along the entire coastal zone, with the cleanest sand and clear sea.
You may like the beach-oasis Jumeirah in Jumeirah-2 (urban complex of beach and park area, entrance for children is free, for adults - 5 AED).
Or perhaps you'd prefer the very popular local Open Beach, which overlooks many local attractions. This modern, super-comfortable beach is free of charge.