beam

of Beam, Emitting beams; radiant.

The beam is the width of the boat at its widest point.

If something is 'on the beam' it is to the side of the boat. The more typical nautical expression for this is "abeam" for instance a buoy could be abeam the mast, ie if you drew a line between the buoy and the mast, it would be perpendicular with the line drawn between the front and back of the ship (bow and stern respectively)

Yes, nautical is an adjective.

An synony for Nautical is deep-sea.