Just came back from Palau with my wife and brother-in-law (July 17-23). It's low season and the weather is hit or miss, but it was great for us--88 degrees/partly cloudy skies, lower humidity than Guam. We went with Sam's Tours booked through MDA in Guam. There's a divemaster there named Jim and we HIGHLY recommend him! He might be away starting this September because he plans to take complete his dive instructor course back in the US. Daniel is another good divemaster, and I hear Joanne is good too, but in reality, we were very impressed with Sam's Tours and everyone were super nice. Irene at the dive shop is great to work with. Sam's offers free EAN32 and they rent all gear including dive computers (Suunto Gekkos).
We didn't see hammerheads, and I don't think hammerheads visit any of the dive sites between Koror and Peleliu with any regularity. We did see a bunch of grey reef and white/silver tip reef sharks, sea turtles, and a manta ray (specifically German Channel). Lots to see at Blue Corner and Ulong Channel. Sam's will provide you with a reef hook, so no need to buy one. There's not much in the way of fish at Blue Holes or Saies Tunnel, but they shouldn't missed. The Flame Scallops are a really need thing to see and the ambiance/setting is quite thrilling.
Expect 1-1.5 knot currents that will shift unexpectedly. Max current of 3-3.5 knots were reported around Western Pelelui, but luckily we didn't experience that! I HIGHLY recommend Nitrox certification if you don't have it yet.
That said, a livaboard may be the best way to dive Palau, but for about half the price, Sam's Tours was great! Included were airfare from Guam, 6 nights accomidations at the West Plaza Malakal (closest West Plaza to Sam's), 4 day three tank boat dives with free nitrox, and voucher's for free breakfast at Sam's Bottom Time Bar and Grill, and included lunch (the bento box was enjoyable, but sandwiches are available). The West Plaza is a basic no-frills hotel. If you want higher class, check out the Palau Royal Resort, The Palasia, or the Palau Pacific Resort. Either way, Sam's will pick you up via minivan or by boat.