To follow up on this, I added the 10mm extension ring, and that fixed my focus problem.
The 15mm isn't happy behind a small dome, quite apart from alignment of the dome with the nodal point or the entrance pupil and physical obstruction of the lens. That said, I've never felt the need for manual focus with a big dome or even with a smaller dome and teleconverter.
The Nauticam port chart recommends either a 4.3" or 8.5" dome. I've got a 10" dome that I haven't used in years because the 4.3" creates less drag when mounted to the scooter. The video I shot after adding the ring was nice and crisp, though, so I think the 15mm works pretty well with the 4.3" dome.
In the PNW, our waters are usually murky, which necessitates getting pretty close to the subject. At those distances, I find manual focus a lot easier, especially when combined with the focus assist mode of the SmallHD monitor.