Rocketeer Star Saves Three Men From Drowning

TMZ has learned ... Billy was sailing his brand new schooner from Nova Scotia to Rhode Island when he noticed an overturned vessel floating in Buzzard's Bay, MA on Monday around 7:30 PM.

As Campbell sailed closer to the boat, he saw 3 men hanging on for dear life ... and immediately ordered his crew to pluck the men out of the water and bring them aboard.

The rescue was nothing short of miraculous -- because the capsized boat had no emergency lights ... and the men had no way to signal for help.

We're told all three men were treated for hypothermia ... but are otherwise OK. It's unclear how the boat capsized in the first place.

We reached out to Billy for comment -- but his reps told us the actor is still at sea and therefore unavailable for comment.