Best option for using mobile in USA

Hey all,

New to community so hopefully someone can advise. 

I'm going to florida in October and I'm looking for any info on using my phone while there. Mainly due to the charges that I may incure while over there. 

I'll be using my phone as a sat-nav also or would I be better off buying a cheap sat-nav with usa maps on.

Any advice would be great, Mainly to help with costs.

Rogerde