I'll second March as the best month for Palau.  The trade winds are still
blowing, but the rainy season is over.  That's also the time when mantas
mate, so you might be treated to a mating dance if you're lucky (guaranteed
if you make a Yap stopover).  Otherwise, I'm not sure of any major
seasonality to the Palau animal life, and the wrecks of Truk are sure to be
in place no matter what time of the year you go.

The trade winds might rough up the surface a bit, but they're really, really
necessary to ward off the "oppresively high" humidity that the quote below
warns of.  I recall sitting around the pool at the PPR one night close to
midnight, with a couple new Aussie friends that had stashed a couple sixpacs
of VB in the bartender's ice chest (the guy, a Sydney vice cop in real life,
looked like a character out of Mad Max, 6'4", all muscle, and a bald head as
shiny as a baby's bottom, so I can understand why the little Filipino
bartender was so agreeable even though the cheap Aussie bastard never tipped
and constantly haranged us Americans for being generous).  We were there in
May, after the trade winds and before monsoon season, but the initial part
of the trip had stayed breezy due to an out-of-season "tropical disturbance"
in the area, Typhoon Hagibis.  It was already warm and humid enough to heat
a cold can or glass of beer to room temperature in under five minutes, but
the wind had kept it pleasant for the most part even though it had prevented
Neco Marine's boat from going outside the cut in the reef to Blue Corner.
That night, the wind finally died.  Sitting there practically motionless,
previous enjoying the beer and the breeze and the antics of the Mad Max
character, I suddenly felt beads of sweat start rolling down my forehead.

We got lucky in Chuuk.  It had rained the entire week before our trip and I
found out later it rained for several weeks after our trip, but we somehow
picked the one dry week in the November after 9/11.  I don't even recall it
being too hot or humid then, but it's usually breezier sitting on a boat
than on land.

Here's a little more info:

"In Chuuk, the climate is chiefly influenced from November to June by the
north-easterly trade winds. By about April, the trade winds begin to
diminish in strength, and by July they give way to the lighter and more
variable winds of the doldrums. Between July and November the islands are
frequently under the influence of the inter-tropical convergence zone. This
is also the season when moist southerly winds and tropical disturbances are
most frequent. The humidity at this time is often oppressively high.
Rainfall averages about 365 cm a year. The most pleasant time of the year is
the relatively dry period from January to March when monthly averages are
less than 21 cm and February is the driest month with an average of 15.7 cm.
Rainfall varies widely from year to year in amount and in seasonal
distribution. Annual totals have been as low as 300 cm and as high as 450
cm. Even in the drier season of January to March, monthly rainfall has been
as much as 60 cm in some years, however, it has been less than 3 cm.
Temperature is relatively uniform throughout the year varying 10 C from the
averages for the warmest and coolest months. "

"Palau's climate is tropical. Rainfall varies from 120 to 160 inches (3,050
to 4,060 mm) per year. Humidity is fairly constant, ranging from 77 to 84
percent, and temperatures vary not more than 10 °F (5.5 °C) diurnally,
monthly, or annually from a mean of 81 °F (27 °C). Northeast trade winds
prevail from December to March, and southwest monsoons from June to October.
Prevailing oceanic currents offshore are the North Equatorial Current and
the Equatorial Countercurrent."