What happened to this ocean?

I believe the ocean biome requirements are met here as there are more than two tiles tall of water over sand and is located at the edge of the map. In other words, it was bad RNG that provided you with an ocean that is not ideal.

You can create the ocean biome there by adding more water to the basin. You can duplicate water by placing blocks in a J shape or ∴ shape. Spam click the water bucket to place water on the hook of the J or the top block of the ∴. I suggest using the ∴ and expanding the pattern outward to allow for more water to be placed at once, speeding up the process.

You will need somewhere around a thousand tiles of connected water for it to count as fishing in the ocean biome.