Out of curiosity, do you believe the probability of loss or theft of your money is higher in Cuba than some other destination? As far as the theft part, Cuba is one of the lowest crime countries I have visited. It's my understanding that crime against a tourist is exceedingly rare. As for the possibility of money getting "lost"--maybe you envision it slipping out of your pocket or something--although that, too, seems unlikely, there are probably some good options for avoiding that, such as a money pouch you wear around your neck. It's not going to get "lost" while you're on the boat.
That said, I can't fault you for taking precautions on your vacation. Better safe than sorry.
