"The last boat's at 7pm, mate," warns the skipper. "But if there are 50 people, and you're the 51st, you ain't coming on board."
We're out in Gravesend to use the most easterly public crossing on the River Thames. It's a half-hourly ferry service that connects this historic Kentish town with an abandoned rail station in Tilbury, Essex. Pedestrians and cyclists only.
The limit might be 50 people, but rarely do so many turn up. Catch the ferry on a weekday, and you can have the boat to yourself. We're here during the Gravesend Regatta, when trade is a little more brisk.
Our boat arrives at Gravesend Pier. It's a tiny thing. The Woolwich Ferry, a few miles upriver, can carry three times as many passengers, plus 40 vehicles. We pay our £3 'off-peak' return fare to a windswept deckhand. Bizarrely, it's cheaper to buy a return than a single. Barely have we taken our seat out on the foredeck, and the boat chugs off.
The fun-sized scale makes the ride all the more enjoyable. Our plucky vessel pitches this way and that as it makes its way across the river. The journey lasts about 10 minutes. There is no commentary; not even a safety warning, although the boat is well-stocked with lifejackets. Part-way across, we're lucky enough to see one of the Tilbury tugs showing off its sideline as a firefighting vessel.
Nobody knows quite how long a ferry has operated on this stretch of river. It is probably medieval, possibly Roman. Needless to say, the vessels have changed many times over the years. Its original purpose was almost certainly to transport livestock across the river. The 17th century ferry was little more than a rowing boat. By contrast, a car ferry plied these waters between 1927 and 1964.
We disembark at Tilbury before we know it. The Tilbury terminal is little more than a wooden shack. More impressive are the huge iron bridges that anchor the pierhead to Essex soil.
This is the spot where the MV Windrush disembarked in 1948. Onboard were the first large group of Londoners-to-be from the Caribbean. Their arrival is marked in the name of Windrush Square in Brixton, an area in which many of the newcomers settled.
Tilbury today is an industrial megazone, given over to shipping, containers and logistics. It's not without cultural interest, though. Visitors can marvel at the nearby Tilbury Fort, a Tudor stronghold now overseen by English Heritage. Grab a drink in the isolated, and therefore suitably named, World's End pub. Oh, and be sure to check out England's least interesting viewing bench.
There's also a ghost station. Tilbury Riverside once hooked up with the line to Fenchurch Street, but closed in 1992. Many of the buildings remain. Oddly, Tilbury Riverside still exists in the National Rail Enquiries database. You can plan a route via this defunct terminus.
You wouldn't want to be stranded here, mind. It's a long, lonely walk around the container port to the nearest actual station. With the captain's warning in mind, we make our way back to the ferry terminal well before the 7pm last boat.
Returning to Gravesend, we grab a final riverside pint in the Three Daws pub — older than anything in London proper. The ferry heads out again with maybe five passengers. Will this ancient and most easterly Thames crossing still be here in another century?