What if I’m fishing in brackish water?

According to FWC:

Whether you need a freshwater or a saltwater fishing license depends on what species you are targeting, not how salty the water is. If you are targeting snook upriver in fresh water, you’ll need a saltwater fishing license.

Which basically means if the fish that your targeting is listed as a saltwater fish, like snook, then you’ll need a saltwater license. If you were targeting tilapia or bass, then you’d need a freshwater license.
Make sure you know which fish you are targeting and have the license for them. This is why the lifetime license was a great buy.