I am looking to switch from my Nauticam D500 to Nauticam Z8. Any thoughts on using the Nikon 8-15 mm vs the Nikon 24-50

I suggest getting both! These are both really great lens setups. While there is some overlap in range they both have unique properties and use cases. With the 8-15 under a 140mm dome, there size is also fairly similar.

The WWL-C with 24-50 is a great all around setup. Many divers would be happy with this as their only lens. It handles standard wide angle down to near-macro subjects. Very versatile and high quality. (don't consider the 24-50 without the WWL-C) It is also easy to shoot and master.

The 8-15 fisheye and 140mm dome are possibly the highest quality image you can get in a wide angle. However it is more difficult to shoot and light your subjects effectively. It can shoot an entire Humpback Whale from 3 feet range, or a 3 inch fish from Zero inch ranges. Extremely wide to Close Focus Wide Angle small subjects. This is a more expensive lens and will take some learning, large strobes, and investment of time to master.