We also have a rescue
on the boat, who we got when he was 2. I'm happy to report that 10,000 miles later, he's doing fine with his mat, but he'll still hold it for quite a long time before resorting to it.
We did it in 2 stages.
First, I bought a piece of sod and stuck it on a tarp in the cockpit
. I brought it with us for a walk on shore and then stuck it under him while he was peeing, just to get the smell right.
Once he'd gone on the sod a couple of times on the boat, we tossed that overboard
and switched to the astroturf. He's never been really comfortable with it, but we have done offshore
passages of up to 9 days and he does use it when he has to. It seems like after he's had access to shore for a few months he sort of has to re-learn again, but it's not as difficult as the first time.
So, that's what worked for us, but every mutt's different. I'd definitely try a piece of sod first, though. That seemed to be a good way to go.