Just cruised on the Dream and thought 'd report back as I couldn't find a definitive answer pre cruise....
The Disney Visa offer is good for any "Signature Service" treatment on port days. My hot stone treatment received the 20% off on our Castaway Cay day but my girls' standard Swedish massages did not. Also, the entire day is considered a "Port Day" as our treatments occured just 15 minutes before all aboard. Lastly, I just needed to have my Disney Visa on my room account to receive the discount. IE, I was able to put the charges on my room to be paid for with onboard credit as opposed to having to charge the treatments on my card. Hope this helps anyone else looking for details!