I usually bring a lot of small bills, 1s and 5s (US) with me as well as some 20s. This is to get me started till I get to the cambio or the ATM to get J dollars. I prefer paying in Jamaican dollars instead of relying on myself to be in the right mind to do conversions--but that said US dollars are taken everywhere and prices are sometimes posted in US and JA dollars. Money is money--so whatever you are comfortable with I think would be fine. You mentioned spending just one day at the beach--if you are only looking at one day and are not using JA anywhere else, than US is fine. Just keep in mind whatever the current exchange rate is or have a conversion chart handy.