Although you might call and charge it to a credit card, there's really no advantage, and the next few days are a lot of fun getting ready, you don't need to worry about that issue.
Also, don't do it when you board! Getting on the ship, lunch, exploring, all are wonderful and magical, you don't need to be in line doing anything you don't have to! You can visit guest services any time during your cruise (when you happen by and see little or no line) and pay anything to your account.
But again, really evan that is really no advantage. The suggested gratuity will be place on your bill and just charged to your card at the end of the cruise. Whether you call now, pay during the cruise, or just let it be charged to your card along with all other shipboard spending, it all comes out the same. Enjoy your cruise, appreciate the great service you'll get from the servers, and just let the tips appear on your bill.