The US -Cayman dollar exchange rates have been fixed for decades at 1.25 US to 1 Cayman dollar. That eliminates all the hassle and games of currency issues like SXM with the Euro to dollar.
I recommend getting some local currency for small purchases. It's respectful, local merchants shouldn't have to deal with credit card transactions and commission fees when you buy some water, soda, etc.
Just back a couple days, love it there. Lots of progress and improvements over the years, except for the awful airport!
Kind of the opposite of SXM.