The Fairey tactical/dive bomber that the Fulmar was modified from first flew Jan 1937. About 10 months after the Spitfire. but ahead of most other retractable gear Naval fighters. (11 months before the Buffalo prototype)
Granted the idea to fold the wings wasn't there yet.
Fulmar had much longer range/endurance than the Spitfire and carried about 2 1/2 times the amount of ammo for it's guns.
Ability of small number of Fulmars to maintain a CAP without frequent landings was an asset to the RN.
You need more than just the idea for a single seat fighter, you need one that will work for the RN in being able to perform other roles and/or perform some of it's roles for longer periods of time than some other countries fighters.
Try substituting A5Ms for Fulmars on the Mediterranean convoys and imagine the results you might get.
The Fulmar may have been under utilized by the FAA. The MK II could drop a 500lb bomb in a 60 degree dive. Since the plane it was modified from could dive bomb (a pair of 250lbs, one under each wing) the lack of bomb carrying ability on the MK I is a bit of a mystery. Weight of guns or lack of power from the Merlin VIII engine??
If you want Seafires in 1939/40 you need to fix a bunch of things along the way. Increased production of better propellers, earlier adoption of 100 octane fuel, beef up the landing gear (peacetime and wartime standards of acceptance are not the same, peacetime acceptance would not tolerate the accident rate that wartime would.) You need another plane to do the long range reconnaissance/scout role.