I had this exact same problem!
I'd snorkelled with a few other excursions but their rental gear had prescription masks for rent. However, as you know there aren't any on CC. I'd heard about the goggle solution too but others had said the lenses are so small it really blocks your peripheral vision.
So I had to figure out what to do. While it is true that vision is better in the water, it wasn't clear enough for me to feel comfortable/safe in the water. So I looked around on-line and Rx masks were in the $150 range - a bit more than I was hoping to pay.
I happened across this amazing site: snorkel-mart.com They had the masks for about $60! I was worried they'd be cheap, etc. but when I looked at their website vs others, theirs was superior in the info it gave in explaining Rx masks, etc. I called to see if I could talk to a live rep and got thru right away. She was VERY helpful and patient and actually suggested I not get some more pricier options I was asking about.
Short story: I bought the mask and some cases (DH has his own equip and it's always bugged me that his mask didn't have a case), even tho I was concerned the stuff might be lousy quality. The cases and mask arrived promptly, are were of exceptional quality (full silicon seal, etc.) - I tested it at home and everything worked great! I was so happy!!
(full disclosure - I am not affiliated with them in any way. I just found them when I was looking for an affordable solution to my problem.)
On the cruise, it was wonderful to not have that stress of knowing that I wouldn't be fully able to see, etc. Even if you don't get anything from Snorkel Mart, take a look at their site under Helpful Info for some education on prescription masks.
BTW, I had my own snorkel and if you don't have one I'd suggest you get one, even if it just a cheapie thing (like mine). The other places I've rented from let you keep the mouthpiece.
DCL doesn't and I think it's revolting, even if they clean it.
Have a fab trip!
cheers,