Word for outdoor area on the side of a ship

Solution 1:

Your question fits the description of promenade deck from Wikipedia:

The promenade deck is a deck found on several types of passenger ships and riverboats. It usually extends from bow to stern, on both sides, and includes areas open to the outside, resulting in a continuous outside walkway suitable for promenading, (ie, walking) thus the name.

A deck does not have to be a large open area:

Deck 1. Nautical a. a floorlike surface wholly or partially occupying one level of a hull, superstructure, or deckhouse, generally cambered, and often serving as a member for strengthening the structure of a vessel.

Solution 2:

Do you mean, "promenade deck"?