Riker later demonstrates that the SOS signal was the work of the Bow People, as was the "Goliath Onboard" message. The SOS signal was set to run on a constant basis out of a naive but forlorn hope that it might be heard one day by outsiders. In that context it isn't a plot hole.
What IS a plot hole is how the fact that these messages came from the Bow People and Riker wasn't addressed until later. Why didn't Selkirk or the others say, "We heard your SOS and this message" which McKenzie and the others couldn't have known about. Unless they knew such messages could only have come from Riker so that they knew it was better to play along and take credit for them and not arouse any suspicions.

All I can say to someone called Christina:
No more hang wires!

‍️ Christina 1986-05-20

Definitely! Except I hurt my back.
My favorite is to jump on a swinging bar! You know, I seriously would prefer to move somewhere Up North where they have a good trapeze school, hopefully there is even a good one.
½ S/N Asian (40%+ Chinese) ½ Norwegian/Danish-Irish Swiss (Amish/PA) German French Dutch? French+Dutch Celtic-Irish English-Irish? ..?

Do you still go to playgrounds and swing?
My password is password

‍️ Christina 1986-05-20

"You don't say…"
½ S/N Asian (40%+ Chinese) ½ Norwegian/Danish-Irish Swiss (Amish/PA) German French Dutch? French+Dutch Celtic-Irish English-Irish? ..?

Without visual surveillance, TV, sonar - there is no way that they could know that divers are outside.

‍️ Christina 1986-05-20

They suspected they would be there, maybe, and sent a signal for rescue.
½ S/N Asian (40%+ Chinese) ½ Norwegian/Danish-Irish Swiss (Amish/PA) German French Dutch? French+Dutch Celtic-Irish English-Irish? ..?

How did anyone inside the ship was monitoring divers outside?

‍️ Christina 1986-05-20

"It would be a kind gesture." Yours Sincerely, David E Barrett
½ S/N Asian (40%+ Chinese) ½ Norwegian/Danish-Irish Swiss (Amish/PA) German French Dutch? French+Dutch Celtic-Irish English-Irish? ..?