Best: W Maldives and its terrific house reef by all accounts

Other SPG resorts with snorkeling at resort or nearby

French Poynesia:
St Regis Bora Bora (short trip to Coral Gardens)
Le Meridien Bora Bora (short trip to Coral Gardens)

Hawaii:
Sheraton Keauhou Bay (adjacent to harbor where boat trips to Capt Cook monument/Kealakekua Bay originate; short drive to snorkeling at Kahaluu Beach Park)
Westin Kaanapali Ocean Resort Villas (good snorkeling right in front at Kahekili "Old Airport" Beach Park)
Sheraton Maui at Black Rock and nearby Westin Maui
St Regis Princeville (Puu Poa is OK but adjacent Hideaways is much better) and Westin Princeville Ocean Resort Villas
Sheraton Kauai at Poipu (OK)

Florida:
Sunset Key Guest Cottages and Westin Key West Resort

Caribbean:
Westin Casuarina Resort, Grand Cayman
Sheraton and Westin Grand Bahama Island Our Lucaya Resorts
Westin St John (not at its beach but there's terrific snorkeling spots along north shore beaches of St John)
Snorkeling at Aruba, St Lucia and St Martin is not renowned

Africa:
Sheraton Soma Bay Resort, Egypt (Red Sea)
Sheraton Sharm Hotel, Egypt (Red Sea)
Le Méridien Dahab Resort, Egypt (Red Sea)
Le Méridien Ile Maurice, Mauritius
Grand Mauritian Resort & Spa, Mauritius
Le Méridien Barbarons, Seychelles
Le Méridien Fisherman's Cove, Seychelles

Australia and Asia Pacifc
Sheraton Mirage Port Douglas Resort (close to Great Barrier Reef)
Westin Resort, Guam
Sheraton Maldives Full Moon Resort & Spa
Le Méridien Ile des Pins, New Caledonia