I've noticed several versions of 3" PVC to hold the lens and be able to adjust the focus. Was getting the boat ready for the water and I noticed the inspection ports. (Actually they're called deck plates). These might be an alternative; they're basically a 4,6,8" diameter PVC plate that's threaded. The whole thing gets mounted to a panel on the boat so you can unscrew it to store stuff inside or just let the airtight spaces dry out.
Point is, you could cut a hole in the middle of one of these to mount the lense in and use the threaded panel for focus adjustment without getting any blockage.

Interesting, any sources or pictures?

I don't mean to be rude, but I'd like to see this stay on topic, showing pics of current solutions ONLY. Not ideas for focusing mechs. The ideas are out there already, we need to have a concise spot to find proven methods.

Sorry Tony; since Brainchild asked I'll reply but feel free to delete or move. A few people seemed to be struggling with 3" PVC and this looked helpful. This is typical of what's available; they come in different sizes, and you could get brass or stainless if you could work the material.

http://store.hamiltonmarine.com/browse.cfm/4,2520.html

Tony, calm down. I'll move the posts once I'm done talking to Dave.

Dave, that's neat. Do you know how much thread is available on these?

They're pretty thin; I'd say 1/2" of thread or so, so just for fine focus.

Ok thanks Dave, not what I was looking for. I want something with about 2.5" of travel.