Palau is most famous for scuba diving. One of the most famous dive sites - Blue Corner, with constant sharks and a high current - is located less than 1 hour's boat ride from most resorts. Many live aboards like Ocean Hunter operate out of Palau. There are also tours to WWII battle fields on Palau.
The Blue Corner, German Channel, Ulong Channel and Blue Holes are all amazing dive sites. You can dive the same site again and again and have completely different experiences each time.
Palau is also famous for its jellyfish lakes. These lakes contain jellyfish which have evolved away their stingers in the absence of predators. There are many tours which will go to the jellyfish lake to snorkel. SCUBA diving is not permitted, nor is necessary, in the jellyfish lake. Palau Jellyfish Lake (http://www.dailyscubadivi...) is included in the category of natural phenomena and scientific mysteries.
Expedition Fleet (http://www.expeditionflee...), is the largest privately owned live-aboard fleet in the Philippines. Their ships operate all over the Philippine Island and Palau. Expedition Fleet is known for experienced and professional Dive Masters as well as excellent service on board.
Splash (http://www.splash-palau.com/), the dive shop attached to the Palau Pacific Resort is recommended. The equipment available for rental is of high quality, and either new or well maintained. The dive masters are also very experienced, responsible and know the dive sites very well. Angelo at Splash is highly recommended as a dive master especially if you have not dived in stronger currents. It should be noted that Splash runs a rather large, wide diveboat, containing 20+ divers.
Fish 'n Fins (http://www.fishnfins.com/) is the oldest dive center in Palau. They currently have two live-aboard vessels, as well as seven smaller and faster! dive boats, operating from the base in Koror. The guides are very professional and are more than willing to share their extensive knowledge of the ocean and the life in it. Divers can use Nitrox EAN 32 for the same price as air. Gas mixtures for technical divers are also available.
Sam's Tours (http://www.samstours.com/) is another dive shop in Palau that offers diving, snorkeling, kayaking, fishing and land tours. They have some great guides that provide educational and environmental information about the locales. Sam's Tours uses small, fast narrow boats which carry 4~8 divers.
Palau Dive & Scuba
Small, personable service that brings together all the information on dive sites, conditions and dive centers for Palau.
Sara Guide Service
Environmentally responsible professional sports fishing guides in Koror. They have experienced Palaun boat captains and Japanese and Western guides that make you feel welcome. Catch and release fish and have a great time! Great food too on their 8 and 10m boats.