Offshore oil will only generate in new chunks, and if a normally-seeded oil deposit happens to coincide with a body of water.
To add offshore oil to chunks that are already generated, you can make a backup of your save and then try using the /regenerate-oil console command. This will add any offshore oil deposits that would have been generated in a new game, but it will also return all oil deposits (land and sea) back to full strength, and also regenerate some biter nests.
