You may see St Mariam Cathedral from a distance before you actually find this grand building. Rest a while inside if it's open or just admire the facade from the street.
You'll have sightseeing choices galore when visiting Massawa. St Mariam Cathedral is an ideal place to begin here, but there are plenty of other worthwhile spots to explore. Imperial Palace, Covered Bazaar and Sheikh Hanafi Mosque are close to the principal church and and will cap off your day nicely.