Lots of bug spray as there are mosquitos, sun cream and bikinis. Also remember US charging converters and your UK ID as you will be ID'd in Florida.
Yes, you need an ESTA. Register here: https://esta.cbp.dhs.gov US Customs now uses an App called MPC which allows you to access 'speedy customs' to avoid the queues. This is worth doing as customs queues can be large in Tampa airport.
Yes, standard tipping in Florida is 20% so make sure you bring cash.
It's a beach wedding so we would recommend sandals or flip flops for dancing.
Your presence is enough. We appreciate the special effort everyone is making to celebrate with us and no gifts are necessary.