You can visit the Carnival cruise line website and it will show you the itinerary for that cruise. For an Alaska cruise, they don't normal have to adjust the scheduled stops so you can certainly find out where they are. BUT sending them a message is not cheap. You can call them shore to ship but it is pricey at $9.50 a minute. They can use their cell phones in port and on the ship (international rates apply while on the ship) but unless they call you first, the odds are the cellular company will not know where they are roaming and could not forward your call to them. The other possibility would be email. Again, unless you know they had planned on using the internet cafe service onboard you may be wasting your time there. The only other thing you could do is call the Carnival cruise line Bon Voyage department (phone number is on their website.) You can send them a gift through them (although I am not certain you can do so during a cruise, I have always used them before sailing) They have many things to choose from. You could simply apply an on board credit to their account and leave a message saying "Have some drinks on me, have a great cruise" or buy them some chocolate covered strawberries etc... They can let you know your options when you visit the website.
Hope this helps.
Carnival Cruise Lines:
Fantasy 90, Celebration 95, Victory 05, Conquest 05, Spirit 6/06, Freedom 6/08, Holiday 7/08, Victory 6/09, Valor 9/09, Dream 6/10, Splendor 8/10,Conquest 8/11,Magic 11/11,Triumph 4/12,Spirit 7/12,Freedom 11/12, Triumph 2/13 - The fire cruise, Magic 1/14, Freedom x 2 6/14
NCL Dream October 2006, RCCL Mariner OTS June 2007, Celebrity Silhouette 2013