|Should my phone GPS just pick up the GPS network coverage, and the app will direct me around Florida, even if we have no cell phone plan?|
I'm not familiar with the app but it looks like it should. You could also test it out by downloading the map for where you are now then turn your cellular data off and see if it works. Also, again with your data connection turned off, you could also try routing yourself somewhere in FL now. Make your destination somewhere you plan to go in FL, make your origin point some place else in FL. See if you get directions. It should work.
If it doesn't you could make a habit of connecting to wifi on the phone before you leave from your hotel, or wherever your staying. You could also find wifi in many coffee shops, donut shops, restaurants etc. But I think that won't be necessary you can just use the downloaded map.
If we want to get a 10 day (actually 14 day) data plan that will allow the Google maps function to work properly (and also allow us to look up restaurants a local sites on the phone) where would I buy something like that?
If the cruise does sell it, they probably do so with a markup. You could order over the internet or you could buy them at a Walmart or buy them a store that sells prepaid phones, either one that specializes in that purpose or they would probably be available at a more general store like a Walmart.