Well, we had a lovely day with gorgeous weather, and Pongo is now officially a sea-dog.
He was Most Unimpressed at first with the concept, and simply refused to get on the boat; it was making loud diesel engine noises and vibrating and generally being an Untrustworthy and Suspicious Thing. Only some particularly enticing bits of Food persuaded him that it was safe to step on. After that he thought the whole thing was immensely interesting...not least because there were substantial snacks and treats around, and two of his most favourite hoomans who enjoy feeding him (these two facts are not unconnected). We went for a nice walk-and-sniff down the towpath at one point (which I think was actually his favourite bit of the day) and the only want to get him back onto the Untrustworthy Thing was to pick him up and carry him (!). But he did like sniffsniffsniffing the air and all the smells going past, and wagging at dogs on the towpath; and yes, he thought that Pub Lunch is an excellent concept, especially with his favourite hoomans-who-like-feeding-him. Very happy to get back onto the boat when the engine was off (I think the vibration spooked him more than the noise) and ended up collapsed in the cabin by the end of the trip.
All-in-all a really nice day out and Pongo is henceforth given the title of Captain P. Pugwash.
Not many photos, but here are a couple!

