I didn't use a CC a single time the last time I was down in Negril. (that said, it wouldn't hurt to have one stashed in your room in case you need it)
My GF and I each budgeted for $100 a day. We ended up coming home with extra money - which we pushed into our next trip.
The airport rips you off with their exchange rate. Bring a bunch of $1 bills with you to tip, that should be all you need until you have a chance to change money on the island. Have you driver take you to a cambio. They have the best exchange rates.
I found it much easier to deal in local currency, as the places you visit are more likely to have change in JA. Also a lot of the places lit their prices in JA, so it just makes things simpler.