Floatplane fighters fell into an interesting niche that wasn't quite what one would consider a "traditional" fighter role. Not too surprisingly, the addition of the floats on the plane cut into performance - for reference, the A6M2-N was about 500lbs heavier empty than the A6M2 and had a top speed about 60 mph lower (270 mph compared to 331 mph). In general, in combat between contemporaries, a floatplane fighter is going to be outclassed by a land-based fighter.

So what use were floatplane fighters? The advantage was in their availability. Floatplane fighters may have been outclassed by traditional ones, but they were still perfectly capable of chasing down nearly anything that wasn't a fighter, and they had the benefit of not requiring an airfield. In the Pacific, where fighting was taking place in far-flung islands that often lacked airfields, this was of huge importance. Seaplane fighters could be deployed (usually with a seaplane tender in support) faster than an airfield could be built, providing air cover for a theater without forcing the deployment of an aircraft carrier. In the case of the Japanese A6M2-N, this led to deployments in the Solomon Islands, Dutch East Indies, and Aleutians.

Largely deployed away from where enemy fighters could be encountered, the seaplane fighters would stand alone as the *only* fighters, making them a huge threat to the real targets - patrol aircraft and patrol boats. Maritime patrol was a hugely important and oft-overlooked role for WW2 naval warfare, doing everything from ASW work to scouting out for enemy ships to search and rescue. They often operated alone on long endurance flights, making them very vulnerable when they did encounter enemy fighters. So by stationing fighters in the area, you prevent the enemy from coming too close with their patrol aircraft.

Outside of the Pacific, the seaplane fighter fell more into a niche that wasn't exclusive to seaplanes. Here, seaplane fighters were still intended to fend off maritime patrol aircraft. The French were particularly interested in this idea for their navy during the interwar period (though I can't find anything explaining their reasoning, French naval strategy at the time was focused on the Mediterranean and thus was likely aimed at Italian patrol planes). Other navies also employed a number of different designs armed with forward-firing machineguns (see the Curtiss SO3C or Vought OS2U), but these were more specialized as reconnaissance and spotting aircraft rather than fighters. Some designs like the Curtiss SC Seahawk really pushed things into fighter territory and capability of firing on other aircraft *technically* existed here, but I can't find anything that details how those guns were intended to be used so I'm hesitant to say that these were intended for similar use as seaplane fighters.

In any case, the real need for distributed fighters on ships came to show itself during the Battle of the Atlantic. Convoys operating far from shore were easy targets for patrol planes. Without any meaningful air defense, a patrol plane could leisurely attack convoys or shadow them and report their course to surface ships or submarines. While a seaplane would be fantastically suited for this kind of duty, the solution ended up being the CAM ships - a Hurricane fighter would be placed on a catapult on the bow. If a patrol plane was spotted, the Hurricane would be launched to intercept and either attempt to make it to a land base or ditch at sea and have the pilot recovered. This program was short-lived, however, as escort carriers were soon available in enough numbers to take over the job.