It was always explained to me viewing through a telescope the ships out at sea were appearing as though they were submerged and slowly rising as they approached the port.
Captains were asked if they were submerged or were doing anything strange. Nope.
The only explanations were either the curvature of the earth (use one hand curved and the other as though it was the ship coming in towards them) or the Martians kidnapped them. Don't get me started as to which is correct. If they ascribe to the second theory, tell them to ask the next little green man they see at the airport.
