I don't know about "deadly hot", the average high temperature only changes about one degree between summer and winter. If you catch it right you can expect some really smooth sea conditions in August, but if there's a storm anywhere within several hundred miles the weather and seas can get real crappy in a hurry. And as noted above, it is typhoon season out here so you never really know what to expect more than a couple of days out. So it's a crap shoot and the weather could go from great to terrible while you're on the plane out. Winter you sometimes get the big seas from the storms up near Japan, but chances of a storm/typhoon are less closer to the equator. Plenty of diving inside the reef, so there should always be a diveable site unless there's a storm nearby. Disclaimer: Not a weatherman.
Edited to add: We schedule all our trips to the other islands for wintertime, FWIW. We did Palau around Christmas and it was fantastic weather. I highly recommend doing a side trip to Yap for the mantas. There's mantas in Palau, but nothing like the action in Yap. And manta mating season is something like Dec-Apr.