In November 1921 the Seal II was one of a number of aircraft sold to the Imperial Japanese Navy Air Service and shipped out with the British-led Sempill Mission to Kasuimiguara.

They were considered by Flight magazine to be "always of substantial construction, a proper boat-built job, a boat that will fly rather than an aeroplane that will float".

[2] Supermarine incorporated suggestions made by the Air Ministry after the company's Commercial Amphibian entered a seaplane competition at Martlesham Heath and Felixstowe in September 1920.

[8] The pilot, who was protected in his cockpit by a cover, was seated forward in the nose with the fuel tanks placed behind him and in front of the other crew members.

[11][6] Following the trials, which were generally satisfactory, the RAF recommended a number of modifications to be made that included moving the position of the fuel tanks from inside the hull to the upper wing, and increasing the size of the rudder and the fin.

He lengthened the hull by 2 feet (0.61 m), modified the tail surfaces, and amended the shape of the wings to correct the aircraft's balance.

[2] Both a Seal II and a Seagull were sold to the Imperial Japanese Navy Air Service when it acquired aircraft after a fact-finding mission in Britain.