The first decision is whether you're considering a dry boat (air filled) or a flooded design. The following is for s dry boat.
The secret is in having a decent design that keeps the bearings dry. Simply put you have the support bearings inside the hull, with a plain bearing, or nylon sleeve as the through hull.
I can't upload a sketch, so try to visualize.
You attach the framework to the hull with the shaft supported by ball bearings, then extending through thd hull using a bushing with rubber or plastic seals on the outer edge.
There will be some weepage, depending on depth and pressurization. If you're pressurizing the boat, then slight over pressure will bleed air out and prevent water from entering. Or you can use a box inside the thru-hull, and feed a small trickle of air so there's a positive outflow.
The basic design can be refined but the key is a between the bearing and a 2-3" sleeve as the through hull.
BTW the biggest challenge will be managing air. Whethet you opt to breath through a mask, try to recruit someone familiar with the physiology of underwater exercise, because it's different than on the surface.
Last edited by FBinNY; 01-17-23 at 03:23 PM.