We just got back from Florida on sunday and we took our car seat with us our for DD (they put it in the hold with the buggy as you board the plane) but ended up hiring a local one as it didn't fit the hire car properly. We tried strapping it in but it was really wobbly and then the guy that worked at the hire car company tried to fit it for us but it just wobbled all over the place so we decided to hire a local one which clipped on the car (similar to Isofix) and also strapped in with the seatbelt. It was much more secure. We ended up just lugging our one from home around with us for nothing.
It will cost us £70 to hire a seat for two weeks, i have seen lots of bad reports on hiring a car seat and that is why we were looking at taking ours. We have discussed buying one when we get there but i don't want the hassle of going back and forth to the hire company, i think we have decided to take ours. Thank you to everybody who replyed.
We always take our own car seats as you could buy them several times over if you were to pay car hire fees.
Technically, you should have a car seat that has be positively approved by the local state. In reality, most car seats are fairly international brandwise, so there is little to worry about. The police are not looking to pull over tourists with foreign car seats.