Guess #1 .. They will land on the lead boat, take hostages then broadcast an intention to harm hostages or otherwise escalate (e.g. sink the boat) if all boats don’t turn around.

Guess #2: Electromagnetic pulse.

Guess #3: LRAD
https://en.wikipedia.org/wiki/Long-range_acoustic_device

Guess #4: Successful poisoning this time around.

Guess #5: Low flying aircraft persistently machine gun the water ahead, closer and closer to the flotilla.

These are guesses not predictions