The two common loons in this video were focused on each other for half an hour, employing various behaviors indicative of anxiety and aggression. One of the birds may be one of last year's resident territory holders trying to dissuade a challenger without resorting to high stakes physical contact fighting (with the potential for injury, sometimes serious) or it is possible that two newcomers are sizing each other up. Rapid, jerky bill dipping with quick placement of faces in water is a displacement behavior of anxious, confrontational loons.
Another confrontation type behavior called splash diving was occurring frequently during this pronged interaction as well. Very different from the effortless submergence of a loon leaving the surface to capture underwater prey, splash dives are exaggerated, usually short duration plunges. Typically when one bird dives all in the group go down so that they are not left alone on the surface - a "sitting duck," that may be speared by the bill of another from below.
Unfortunately, neither of last year's resident breeding pair were leg banded so it is difficult to impossible to ID them if engaged here (or when 2 other loons joined this pair later).
Ice melted off Mt. Desert Island's lakes about 10 days ago creating the opportunity for loons to move from the ocean to potential breeding territories on lakes and larger ponds. The next several weeks will involve a lot of conflict on many territories as past residents defend and challengers try to take over.
FYI - as I was filming this a truck with a trailered boat was backing down the launch ramp so you can hear that in background.