Hold up, how do I make water transparent?

[Joswano]

In SMBX2, the one I'm using, water appears as a background object, which means its layer is over the background and under npcs and tiles. But I've seen some levels and screenshots with transparent water. How the heck do I do that? Is there some sort of special thing in the properties for the water background category? This would really help with the design of my levels!

[ElectriKong]

Use a custom BGO using a BGO slot where the BGO appears in the foreground.