In the forthcoming blockbuster Troy Brad Pitt, playing Achilles aboard a Greek trireme, asks “Where’s the rest room?”

To this unscripted query, unlikely to make it past the film editor’s scissors, a Thames waterman, Paul Prentice, pointed the megastar towards a rather large bucket. It is discreetly stowed out of the camera’s gaze, but otherwise it is all that could be expected of a ship’s “restroom” of 1193BC.

Brad Pitt plays Achilles in Troy, which is directed by Wolfgang Petersen, and is scheduled for release next May.

Pitt’s co-stars include Sean Bean, Orlando Bloom, Saffron Burrows, Brian Cox, Julie Christie and Peter O’Toole.

Mr Prentice and his brother Bob were among a party of five licensed Thames watermen who, along with rowing coach Bill Blunden, have spent 12 weeks filming Troy in Malta. The Londoners’ job was to train 90 extras to row a couple of 35-metre Greek triremes without damaging themselves or anybody else.

“We had wanted to take out a team of 60 Thames watermen to do all the rowing for the film, but we had to settle for a team of six to train local extras,” says Bob Prentice, adding: “None had ever pulled an oar in their lives.”

It was more a change of scale than anything else for Mr Prentice, who teaches young beginners at the Poplar and Blackwall rowing club on the Isle of Dogs.

He and the other five watermen on the Malta assignment are all holders of the Doggett’s Coat and Badge, awarded to winners of an annual rowing race between London and Chelsea Bridges. His work on the river includes skippering either the Sarpedon, Salient or the Suerita. These three 35-metre pleasure craft belong to the Crown River Cruisers and ply between Westminster, Greenwich and the Thames Barrier Rowers who answered the call in Malta ranged from teenagers to men in their 50s. Some had to be laid off as they were not up to the job, so throughout the 12-week shoot, newcomers had to be trained.

The modern triremes had twin engines as well as square-rig sail and oarsmen. Occasionally all three were in use at the same time. The original boats were built to ram enemy craft, but the trick during the film shoot was not to hit each other or any of the camera crews.

Sometimes the boats had to be taken to the other side of the island to shelter from high winds, and other occasions had to be beached so they could be seen in background as “Greeks” and “Trojans” slugged it out on the beach.

Thames rivermen are divided into watermen, who ferry people and lightermen, who move cargo around the river, although some qualify as both.

Mr Prentice says that, Troy and Malta apart, he and the other 525 fully-licensed watermen are involved in another historic battle on the Thames.

Like many watermen, he is dismayed by proposals to move to a new system of training and licensing which some view as a recipe for disaster on the river.

The Government aims to harmonise licensing in line an European Union directive on “boatmaster licences”, which in theory would qualify Thames watermen to work on other waterways in the UK and throughout the EU, and vice versa.

The proposed licences, watermen frequently argue, will not ensure the local knowledge and experience required for public safety on the Thames, which has two 25ft tides a day, and is increasingly busy.

Watermen cite the 51 lives lost in the 1989 collision between a pleasure boat, the Marchioness, and a dredger, the Bowbelle, under Southwark Bridge.

The risks on the river are now much higher, watermen say. There are more and bigger boats on the Thames since 1989: the Marchioness was carrying 121 people, near her capacity: many Thames pleasure craft now carry 800.

Responsibility for training and licensing, it is proposed, will be transferred to the Marine and Coastguard Agency.

For the moment, at least, watermen (there are some women too) are trained and licensed by a City livery, the Company Watermen and Lightermen, acting as the agent of the Port of London Authority.

A full licence requires work with a designated river employer to be balanced with courses at the National Sea Training Centre at Denton, near Gravesend. These include chartwork and seamanship, diesel and inflatable craft, first aid, firefighting, and VHR radio.

George Saunders, licensing and training officer with the Watermens’

Company, said: “We now have 180 men and women training to be watermen, the highest since the end of the Second World War, when it sank below 10.”

Most are apprentices, but there are more and more “adopted members”, mature students from many walks of life and some in their mid-50s, all attracted by the huge growth in the number of piers and commercial pleasure craft on the Thames.

Triremes are optional, but all students must pass the same courses and log five years’ work experience on London’s river.