Air not spawning problem

How do I spawn a structure in water without getting waterlogged?

I am making a pirate ship structure and I got them to spawn perfectly on the ocean, I am have one big problem though. The pirate ships have air space under the deck for storage, and when the ships generate, the air is replace with water. What can I do to prevent this?

I've tried using structure voids, which literally don't work at all.

What procedure can I use?