In most cases, US citizens living abroad can vote absentee in both federal and state elections (the last state you resided in prior to leaving the country).
Follow the prompts here for more information on what to do in your particular state.
If you are getting ready to cruise it is a good idea to get registered for absentee voting prior to leaving. You can register absentee with a Federal Post Card Application. That way if there are any problems, you'll know about them. Even if you won't have a mailing address while traveling, being registered as absentee allows you to go online and print a paper ballot, at least for the federal elections. In some cases (El Paso County, CO for example) you can even vote absentee online.
I voted via mail in ballot a few weeks ago but I'll be voting online when possible now that I know I can.