Thanks everybody for your replies!
I had the bus/metro route mapped out. However, checked with the cruise ship for their transport as I was worried about the amount of time it would take for my do-it-yourself technique. Decided $20 was my go-nogo point. Turns out the cruise ship (Celebrity Century) transport to MIA was $14.80! Was able to purchase it a couple days before we arrived. So I opted for that.
Incidentally, the 14 day Barcelona to MIA trip was delightful.
Romelle