We were out in the states on a cruise and stay last Nov/Dec and the rate for sterling was excellent then - we were getting approx. $1.9 to the £1.00
Most people use credit cards - we certainly did with Carnival as this is used as your sail and sign card and when I finally paid the bill for the week, there was no extra 3% surcharge added to my card either.
Regarding whether you should do it here or there is up to you as the rates can change daily. We took some $$$$$$$$$$$$$$ with us and as I said used Credit Card. Most of our purchases came in at something around 1.9 or very close to it so the cards worked out well and of course, you do get the insurance element fixed into that as well.
Hope this helps and you have a good time.