This is directly from a travel site concerning when is the best time to visit Tahiti:
When is the best time to visit Tahiti? As for ships, you might want to take a look at the Celebrity Millennium - it's only a 15-day one-way cruise, but a very nice ship.
Actually, it depends on what you want to do and to see in Tahiti and her islands. The climate is devided into two main seasons : a wet season between November and March and a dry season from April to October .
The wet season is characterized by a warmer and more humid weather. But obviously, the lagoon temperatures are also higher, there are huge rollers to surf, you can attend “springtime” for nature…
The dry season is characterized by a cooler and less humid weather with some sporadic short rains that are often welcomed since droughts and water rationing may appear in some islands. During this period, the underwater visibility is the best for scuba diving, humpback wales are migrating from Antartica and make a stop in our waters. The very famous Heiva (traditional festival) is also organized during this period.
Obviously, as the climate is tropical, rain may appear at every moment of the year and besides, since the Polynesian islands are spread over a surface of 4,000,000 km², the temparatures and humidity are very different from one archipelago to another.
For a longer cruise, take a look at the Sun Princess - goes from Sydney to San Francisco by way of Tahiti and Hawaii or the Dawn Princess doing the reverse.
The Holland America Statendam goes roundtrip from San Diego via Hawaii and Tahiti on a 30-day odyssey, which I think is one of the nicest options.
You could think about doing a one-way cruise, in which case you'd have to fly one-way, which is not a cheap flight depending on where you're flying into or out of. Or you could do two one-way cruises either on the same cruise line or perhaps two different cruise lines if the dates work out. Or you could do a roundtrip from the U.S. and spend alot of days at sea. Many things to consider.
There are some other cruises, but perhaps this will give you some ideas. There are also other cruise lines, such as Regent and Seabourn, but the prices are considerably higher.