If the prototype hadn't been released we would've never known about these domes. I always thought the standing air gulp sprite was just a way to get air bubbles without jumping and it was cut cause it was unnecessary.

Now it suddenly makes sense. Wonder what else there is in previous builds?

Maybe... or maybe it was just meant to be a decoration and they didn't fill the bottom part. Your theory is more interesting though

I made a precise screen shot recreation next to one of the same era in the final for comparison.

The fact that combining six of the water surface blocks made an exact fit confirms the theory in my mind. This sprite could've been activated automatically by looking up when the water is that high but we have no way of knowing if the dome filled up completely or only to a certain point.

Looking at the map, these areas appear at regular intervals. With this being the only air source it would take timing & skill to get from one to another. If you got stuck, you're screwed. I believe it was more likely dropped due to difficulty rather than technical limits.

I think the standing air gulp sprites would've been used when the water level in the domes was around at the bottom of Sonic's head as he gasps for more air before he's back underwater.

Dude, this is a revelation. All evidence points toward your theory being true. I don't see what else it could've been meant for.

I wonder how the unused standing air gulp sprites would fit into this?

I've noticed that the domed areas with brick walls still exist in the final, albeit lacking the brick walls.

I have a theory regarding these brick walls. They would have acted as air bubbles for Sonic to regain oxygen in. The longer Sonic would stay in one, the water level would rise until the dome is fully submerged. This concept was dropped due to hardware limitations and replaced with air bubbles.

Rough mockup of what this could have looked like: