I'm wondering if it's possible to create a .mod
group for boating?

No need, there are several already. Most of the people like yourself who are
unhappy with the rec.boats format have relocated to
groups like boatered.com, thehulltruth.com, iboats.com, etc.

It is interesting that some of these moderated groups have huge numbers of OT
posts as well. They put all the OT posts together in the same folder, and try
to enforce some rules against personal attacks and name calling. If your only
interest is to zero directly in on why your BrandX outboard starts cutting out
when it has been running for a couple of minutes, you can go straight to that
department, get your information, and get out.

Those who pine for a moderated group have several to choose from. Good luck.