As others have said, all travel agents should be charging the same price for your cruise. The only difference will be the OBC that you may be offered.
I use
Dreams unlimited too for my Disney cruises. I have moved dates numerous times and changed cabin requests and there are no charges for doing so.
Next year we are trying Carnival. I had trouble finding a US travel agent, as they can't sell flights for Non US cities. My next cruise is in the Med. we decided to fly in the same day as the cruise boards, to save money, which was offset against having a balcony cabin

. I have always wanted to try one just once.
Anyway, Carnival have changed the itinerary of our cruise since booking, and they are now going to Greece and Turkey, neither of which I fancy and we have lost Monaco and Majorca.
Because I have booked through a UK travel agent, then I cannot cancel the cruise. Buried in the fine print in Virgin Cruises terms and conditions it says more than three port changes are considered a major change. So if we want to cancel, then I will forfeit the deposit. Had I booked directly with Carnival I could have received a full refund.
So lesson learnt by me. I will never book a cruise with a UK travel agent again.
When I went on my Disney Med cruise, I booked everything myself, but I did have the reassurance of flying in the day before the cruise, as we had a post cruise stay. My cruise itself was booked with Kelsie at Dreams.
At least if you book your Florida cruise with a US TA you can always cancel and rebook with another operator up until final payment.
I did moan a bucket full to Carnival about my experience with Virgin and they have been really helpful, so thumbs up to them

.
So please bear my experience in mind if booking with a UK TA. If it's the ship that matters to you and you don't mind where you go, then that's not a problem. But if it's the ports that matter, then book with a US TA.
Of course I do release that changes happen to port schedules, but the ship isn't sailing for another 9 months. I would expect changes Once i was onboard due to a hurricane or bad weather, or even the floods in Venice. As I said to Carnival, if you get on a plane that's going to Orlando and you end up in Turkey you wouldn't be very happy.
Edit - if funds allow go for the longest cruise possible, three days isn't enough. Try a week! Don't forget to re-shop your next cruise onboard to take advantage of the larger OBC and smaller deposit. You can always change the cruise later.