[1.16.5] Adding mob spawns to shipwrecks (and structures in general)

I have the following subscribe event in my StructureSpawnHandler.java:

@SubscribeEvent
	public static void onStructureSpawnListGather(StructureSpawnListGatherEvent event) {
		if(event.getStructure() instanceof ShipwreckStructure) {
			ExampleMod.LOGGER.info("\n======\n Found Shipwreck, Adding Drowned\n======\n");
			event.addEntitySpawn(EntityClassification.MONSTER,  new MobSpawnInfo.Spawners(EntityType.DROWNED, 100, 4, 4));
		}
	}

And I register it as follows in a static block, with my other subscribe class registers:

MinecraftForge.EVENT_BUS.register(StructureSpawnHandler.class);

However, Drowned's do not appear to spawn in shipwrecks when I load new worlds. Why might this be? I've read the Forge classes regarding the StructureSpawnListGatherEvent class, and the above should achieve my desired behavior.