The Museum of the Sea is named for King Carlos I, who used Cascais as his base for systematic oceanographic research in the years before he was assassinated in 1908. The collection covers Portuguese seafaring, traditional fishing techniques of the Cascais coast, ship models, and the king's own marine specimens — corals, dried fish, shells, and his hand-annotated charts.
It is a small museum on the seafront, less polished than the bigger Lisbon institutions but more intimate, and well worth half an hour if you have any interest in maritime history. The aquarium across the road, which displays local Atlantic species, is included with the same ticket.