If your Map is already Generated, its hard.
I´m not sure if its still in. But TC hat a Command to Recalculate Biomes at the introduction of the Anvil Map format. (Hope I get the nam Correct, it was a long time ago).
If you were to Replace the Deep Sea biome in your World Config, with the same Settings, it might Generate at the same Spot and then you can do that.
But that probably wont work, there is only a slight chance.
If you can make a new map, you can just as Easy rename the Deep Ocean.bc change the BiomeColor and add it as Custom Biome, and let it Generate instead of Deep Ocean. (You need to kick out the Deep Ocean as Isle Biome iirc).
OceanMonument Spawn can be added to any Biome.