Ahoy Jonathan!
Your 
high seas adventure is SO close, you must be getting excited.  In my opinion, adding a day to any Disney vacation is a for sure YES! And, the good news is you can likely upgrade your single-day theme park ticket to a 2-day ticket with ease!
Many times you can handle the upgrade yourself online. Simply log into your 
My Disney Experience account, head to Tickets, Passes & Memory Maker, and find your ticket. If upgrading online is an option, you’ll see a handy “Change Ticket” button. Easy, right?
If that button doesn’t pop up, no worries! The super-helpful Cast Members on the 
Existing Tickets phone line are just a call away. Or, if you’d rather wait until you’re at 
The Most Magical Place on Earth, stop by a theme park ticket booth, and they’ll take care of the upgrade for you in person. 
Bon Voyage!
Anne