Jumully

I think Matt discussed this once on a live stream. If you want to chance it, you can cancel your booking right before final payment is due and then as soon as it becomes available again re-book it with a non-refundable fare. There is a chance someone could snipe your room between the time you cancel and rebook. So, it's probably not worth it to chance it. But it could be done.
The safer way to do it only works if there is another set of rooms still available that is satisfactory to you. Book the two new rooms at the non-refundable rate and then cancel the refundable rooms. Thats a lot of work though.

Many people do what you are suggesting but I don’t think it always works out.  You are basically “cancelling” your refundable dep reservations and re booking as non refundable.  My experience has been that non refundable pricing goes up over time and you may find that as you get closer to final payment.  
 
i would also recommend keep watching the prices for the refundable and if it drops you can get it adjusted.
In the end, it’s still probably safer to keep it as refundable and see how it goes.  

If you decide to do it on your own, be sure you take your passports, credit cards and cash with you in the unlikely event you don't get back in time to get back on the cruise ship. 

I did this exact same thing in the 90's - multiple times .. sorry but kids have prevented me from doing recently.  The trick then and I'm betting still now.. be one of the first off the ship.  I always caught the ferry on opposite side of bay from cruise ships.  Cheaper route:  take taxi to Red Hook Ferry, take ferry from there.  Shorter Ferry trip..only 20 minutes to St John. You WANT to be on one of the early ferries..   When you arrive in St John there's always taxi's / pickup trucks that will ferry you to the beach you wish to visit.   I went to Trunk Bay and Cinnamon Bay..   it's so peacefull being one of the first one's there..  Remember, it takes a while for the excursion trips to get organized.  Ferry runs regularly and never had a problem getting back.. oh, and if you want, you can spend a little more time there if you're not into shopping in St Thomas.  So pluses:   Cheaper, there before excursion crowds, take your time.. maybe go to a different spot than the excursion groups.  St John's is gorgeous .. 

Calling @Matt since I think he did St. John’s on his own.  We did an excursion which as awesome as we got to see some plantation ruins.  I was nervous about the ferry so we did the excursion but now I’ve been there I will try going on our own next time!  Jane

Hi Jumully, I think I found what you are looking for? Port maps are https://www.cruisemapper.com/ports      Then type in the port you want. I’ve  read that Costa Maya Mexico was a difficult port to get your bearings too & had searched quite a bit to find this website.