Some of the best white sand beaches with crystal blue water I've ever seen are in Antigua, Bahamas, Jamaica, Mexico, and Turks and Caicos. One big important caveat is that time of year matters. Even the best beaches can get a bit murky as the summer storms roll in. If water clarity is important to you stick to vacation between December to May in the Caribbean.
The Bahamas is sometimes overlooked because it's so close the USA. It also doesn't have a lot of all inclusive resorts. If you want a true breathtaking experience, I suggest heading to the smaller islands and enjoying the small beachfront hotels. If all inclusive is really important to you, check out Sandals Emerald Bay and RIU Palace Paradise Island. One note about Sandals Emerald Bay - The beach and golf course are magnificent. The reviews on the dining and service are very mixed.
Jamaica has great beaches. One famous one is Seven Mile Beach in Negril. It's about an hour and a half from the airport. My favorite resort in Negril is Azul Beach Negril. Sandals Negril and Couples Negril are also good choices. If you want to explore other parts of the islands, Sandals resorts did a great job buying up some of the best beachfront locations. Check out: Sandals Royal Plantation, Sandals Montego Bay, and Sandals South Coast.
The eastern coast of Mexico has the best beaches in the Hotel Zone of Cancun, Playa Mujeres, Tulum, and Playa del Carmen. I did whole video on the best all inclusive resorts in the Hotel Zone. Other great resorts are Secrets Akumal, Playacar Palace, and Planet Hollywood. Keep a close eye on Holbox. This is an island about 2 hours from the Cancun airport. It's the next hot spot with more resorts opening this year.
Lastly, Turks and Caicos offers terrific beaches. The biggest resort there is Beaches Turks and Caicos.