The new Florida Building Code is now live. This is the 7th Edition/2020 version of the Code. There are several changes pertaining to Underlayments in the new Code. The 3 options you have for Underlayment Installation are:  
1. Entire roof deck covered with a self adhered underlayment such as MFM Ultra HT
2. Joints of roof sheathing taped with a minimum 4" wide self adhered tape such as MFM Roof Deck Tape then covered with either a synthetic underlayment or approved felt.
3. A synthetic underlayment or approved felt installed and half lapped effectively giving you 2 layers of underlayment.