I just returned from Sandals regency in St. Lucia and can answer the basic questions for you.
US curency is common, but they use the EC (Eastern Caribbean) dollar.
The $EC is pegged to the $US at the rate of 2.7 - 1
ie: $10.00 EC is equal to $3.70 US
Since the exchange is hard-wired you can figure out the cost the money changers will charge you pretty easily.
There are ATM machines at both St. Lucia airports, as well as most hotels will change Can or US money for you.
You should be able to call your hotel to confirm.
They do not generally except Canadian at the stores though, so you will need to bring at least some US money for the initial tips or purchases.
PS. The porters will try to help carry the bags as you clear the customs gate, but you are really walking about 25ft through large doors to immediately within the Hotel Meeting area. So if you have packed light you can skip this initial “service” If not they expect $1 per bag.
Have a good trip,