Here's just some of the suspicious details from CBS:
Police officials determined that the men were not authorized to operate the boat and had not obtained permission to dive. According to Polish media reports, only one of the men had a Spanish passport, while the others offered only verbal identification.
Another wrinkle was their explanation: The men claimed to have been searching for amber. While the Baltic Sea is famous for its vast deposits of amber, searching for it in the dark is unlikely to be a successful strategy.
Seasoned amber hunters interviewed by Polish media said one more thing didn't add up: The men had an underwater scooter, used to drag divers swiftly through the water — something that wouldn't help in a hunt for small objects on the seafloor, especially as such a device's propeller lifts debris from the bottom, decreasing visibility.
Officers apparently saw nothing suspicious at first in the fact that the men were diving near critical infrastructure at night with no permit and atypical amber-hunting equipment, and the local police did not pursue the matter, releasing the men without further questioning.
They have all reportedly left Poland.
Guesses include smugglers and failed false flag operation.