Still using the Sidi Couriers that I started off roading with with -they've been brilliant and some of my mates have been through 2 pairs of other boots in that time.
Not sure they're absolutely waterproof?. . . but with Sealskins fine.
Not the cheapest but you get what you pay for.
I've not yet come across a proper off road boot that is 100% waterproof.
The nearest to this I've come across are what I use as my road boot, Sidi Adventures. They are fully waterproof (Gortex) and have a proper grippy sole, not the usual flat MX sole and prety much the same amount of protection as my AStar MX boots - not cheap though.
I too have Adventures for the road -don't feel quite as robust as the Courier to my mind (and more expensive) but with more than enough protection for a man with your level of expertise!