I heard that I shouldn't use @SideOnly. So that was my way to register the item models:

	@SideOnly(Side.CLIENT)
	@SubscribeEvent
	public static void registerRenders(ModelRegistryEvent event) {
		registerRender(item1, 0);
		//...
	}
	
	private static void registerRenderMeta(Item item, int meta) {
		ModelLoader.setCustomModelResourceLocation(item, meta, new ModelResourceLocation(item.getRegistryName(), "inventory"));
	}

I changed it and now I have this:

	@SubscribeEvent
	public static void registerRenders(ModelRegistryEvent event) {
		BetterDiving.proxy.registerItemRender(item1, 0);
	}

And in my client proxy the registerItemRender looks like this:

	@Override
	public void registerItemRender(Item item, int meta) {
		ModelLoader.setCustomModelResourceLocation(item, meta, new ModelResourceLocation(item.getRegistryName(), "inventory"));
	}

In my server proxy this method does nothing.

So am I registering the models the correct way now?

Yes I'm using @EventBusSubscriber. But I didn't know that I can specify a Side there. Will do that right know. Thank you