The investigation at the Miami harbor arranged for an agent to find and verify workers appearing on the surveillance footage. Jimmy did not notice anything suspicious from the tapes, so there was no point wasting the supervisors' time for a first-hand observation. However, they had to personally investigate Harriton's island, as it was the most probable place where the yacht had been rigged with a bomb.
The stay at the Puerto Rico Port wasn't long, but the time spent on the island was significant. Installing something at specific locations on the vessel would require time, and the island also allowed them to confirm whether their target had boarded the yacht. The only question was why they waited until they were offshore from Miami to detonate it, which seemed somewhat irrational.
Jimmy had some conjectures, but it was necessary to see the site first; everything was purely speculative for now.
