I was on the OCII in January-Feb last year (2007), but it was a special trip to the outer islands of Yap. Assuming you're going on a normal itineary within Palau, you can expect round-the-clock temps in the mid- to high 80's, with water temp in the low 80's. Occasional showers always possible during the day.
If you have time, try to get to Tebang's wood carving shop...best storyboards in Palau. The several museums are all worth a visit. For an over-all souvenir, I recommend Mandy Etpison's two-volume opus on Palau...the most beautiful book ever published on a small Pacific country (you can find it at the Etpison museum).