[1] The E9W was a two-seat, single-engine, twin-float, unequal-span seaplane designed to be easily dismantled for hangar stowage on a submarine, capable of being reassembled in two minutes 30 seconds and disassembled in one minute 30 seconds.

Following successful testing of one of the prototypes on the submarine I-5, an order for a production batch of 32 aircraft, designated E9W1, was placed.

[3] E9Ws left the Watanabe factory with an Alclad coating and a black engine cowling.

Combat units then went on to apply camouflage as they saw fit, usually from stocks available in depots, shipyards, or other bases.

[2] Although it was in the process of being replaced by the Yokosuka E14Y monoplane, it was still in front-line service at the time of the Japanese attack on Pearl Harbor, remaining in service until July 1942, being used to direct their parent submarines onto Chinese ships attempting to pass the Japanese blockade of the South China Sea.