The South Pacific is generally reasonably calm, however rough seas are pretty much a crapshoot. Except for a few places that are always notoriously rough, the height of the seas is mostly dependent on weather, wind, and season. The Caribbean and Alaska's inside passage are also generally calm, but there are days when it can be a wild ride there too... We were in the Caribbean in December and got hammered for two days straight on the trip north from Costa Rica to Mexico. They will do their best to sail clear of it, but sometimes you just can't get out of it.... you can run but you can't hide...
Size of the ship can affect it too... although still a big ship in the grand scheme of things, the Tahitian Princess is one of the smallest among the new mass-market ships - - 30,000 tons. On a larger ship you tend to feel less motion.
One thing that the Tahiti cruises have going for you is that they are port-intensive, I think there's only one or two sea days, so you're stopped a lot and if you're seasick, at least it won't be for an extended period of time.
One thing is for sure however, if you're worried about being seasick, a trans-Atlantic crossing is definitely not the place to go... the North Atlantic is one of those notorious places... and five straight days at sea with no place to go would not be good if you're seasick.
Don't ever have an empty stomach is the other piece of advice that a lot of seasick experts (including my wife) follow with success.
Good luck whatever you decide!
