World | Ships | Finances | Hubs | Shop. Hidden on the Route Assignment screen.
The Ships tab shows a black dot (every alert dot on a button is black with a thin pale ring, to show on the red buttons) while any ship needs the player: lost with no recovery boat on the way, held in port, waiting at a canal for toll money, docked and paused, a docked cargo ship with no route, or upgrade points to spend. Its tooltip counts each reason and names the ships ("1 held in port (Tundra Godwit)", up to 3 names then "and 2 more"). The Ships screen filter "Needs attention" shows just those ships.
The Shop tab shows a black dot while any ship can be bought (unlocked, a free slot, and affordable); its tooltip lists them.
Maritime Empire