I agree with SonnyV. You can use Yahoo on the ship for e-mail, etc, but also keep in mind that the ship receives it's internet service via satellite. Which can, at times, be quite slower than what you might be used to. So if its overcast, expect to have the "instant" part of IM taken from you.
And to his point, at $.50/minute, I'd just e-mail and check back later. You'll be amazed how those charges add up.