It was a dramatic day at sea for "Below Deck" when one of the show's transport vessels caught fire on the open waters ... and we have the scary footage.

Folks who were there say the boat started going up in flames Friday in Mo'orea ... an island near its more famous neighbor, Bora Bora.

The boat fire looks and sounds super frightening ... but the good news is we're told there have been no reported injuries.

It's unclear what caused the fire.

Silver lining, the boat on fire is not one of the expensive luxury yachts you might recognize from "Below Deck" ... it's actually a tender boat, a smaller vessel that ferries passengers from the mega yachts to shore. Still, tender boats ain't cheap.

Boat ... break out another thousand!!! And then some!!!