How to remove grey base from asset?

[Shroomblaze 320]

Does anyone know how to remove the grey base wall you get when making an asset that gets placed on water? In this example you can see the Yacht has it's base removed... I tried figuring out a way through mod tools but I can't seem to get it to work. I even used that Yacht as a template in the asset editor but I still get the grey wall you see below the cargo ship. Could someone please point me in the right direction? Thanks