I wouldn't worry about it. Turks and Caicos is pretty first world. Chances are if you need fuel it will be in Providenciales where they are at least 4 fuel docks and maybe more now. Ask the sport fishing guys. They fuel every day they go out.
The only time we have taken a sample was in Panama. I pulled out a peanut butter jar to check for water and the guys on the fuel barge starting laughing. The mate came out with 2 sample bottles. He dumped couple of gallons in the barge and then filled the sample bottles; one for him and one for us. We felt like a big ship.