The 55 gallon drums in these boats were not full of fish.
The drums are full of fuel so the boat can either get to the next island in the chain, starting with Aruba, and unload or reach a pre-positioned boat with more fuel to get to Puerto Rico/the Florida Keys. The drugs are wrapped in waterproof bales.
Watch the Discovery Chanel's "To catch a smuggler," this season, episode 2 to see how these boats were sometimes intercepted prior to 47. They show the drug loads from the few they were able to catch, even with a helicopter some got away.