They would have a number of options, only one or maybe two of them pleasant.

The first option would be to land on another carrier. Of course, this presupposes that there's another friendly carrier to land on, which isn't exactly a guaranteed thing. For examples of this, see the Battle of Midway, where planes from the Yorktown landed on the remaining two American carriers.

The second option would be make for a nearby base. Again, assuming there's one available. It was not terribly uncommon for carrier battles to occur near islands, some of which may have friendly airbases.

Option 2a would be to ditch near friendly ships. Doesn't have to be an aircraft carrier, any vessel with your flag on it will do. That way you could be picked up and (eventually) wind up back in the cockpit again.

The Third Option would be head for an island... any island. Your carrier has sunk, you're low on fuel, there's no runway nearby... you bet you're headed for a landmass. You may not be able to put the plane down without breaking it, but it's better than Options 4 or 5.

The Fourth Option would be ditch in the ocean while you still have fuel. This will allow you a certain amount of control over the ocean landing and even let you decide to try again if you don't like your approach the first time. If things go well, you'll be able to get out of the plane alive, unharmed, and with your life raft and emergency supplies.

The Fifth Option is ditch when you're out of fuel entirely. This is like Option #4, except you don't get a second chance, you're going into the drink now, whether you like it or not. If things go well, you're in a raft with food and water.

Options 3 isn't automatically a good thing. Being on a Pacific island, alone, possibly injured, has the advantage of not being in the water. In theory there should be food and clean water available. In practice, this may not actually be the case. The island may not be inhabited, or it may have residents that aren't fond of outsiders. One can imagine what happens after that.

Options 4 and 5 are the worst possible, and in a lot of ways the most likely. The Pacific ocean is very, very, very large, and a life raft very very very small. If you were a Japanese pilot, your chances of being rescued were practically nil. If you were a member of the Allies, your chance of rescue was... better than nil. The US Navy actually did search and rescue with their PBYs and small craft like PT Boats. Later in the war, they would station submarines off the coast of... whatever island the Navy was beating up on at the time, up to and including Japan itself... and pilots would know the general location of them. If you put down near one, chances were good you'd be rescued.

But if you put down in the Pacific, there was a good chance that nobody would ever see you again. Even in the best case scenario of ditching... life raft, uninjured, rations... you may very well just disappear into the ocean that covers 1/3rd of the surface of the Earth. Or maybe you'd make landfall somewhere... see Option 3. Or your rations run out, it never rains, and you get the worst case of sunburn ever... and die in your raft. Or the storms come, dump you out of your raft, you lose your supplies, and you die in the water, maybe eaten by a shark.

Obviously, if you are injured your odds of survival drop in all cases.

As you can see, not having a place to land is A Very Big Deal.