G'Day Lori,
I believe that Truk isnt really seasonal. Same as a lot of dive destinations you just have to cross your fingers and hope for the best!
I heared reports back about 1 month ago that the vis there had been 10-15m. Which when diving the big ships isnt really that good. But in saying that, how can you pick that? You can't. Last trip our shop took, we had over 30 -40m vis and some of the best diving some of the guys had ever done.
The same thing happened with Bikini last month. I had some mates diving there and the vis was quite low as well. You win some, you loose some.
I hope this doesnt muddle your planning, but i hope it helps.
Yours Truly,
Carl Fallon