What about focusing on some of the other groups that use the Thames - they might be happy to help you out ?
For example, the London Division of the Royal Naval Reserve is based at HMS President, near Tower Bridge overlooking the entrance to St Katharine Docks. It is home to over 370 officers and ratings, and is one of the largest Royal Naval Reserve units in the country.
The Unladylike Ms. Jo [on this forum] is also involved with another group that occasionally go messing about on the river, but I can't remember which one at the moment.
Or there's the Tenacious Jubilee Sailing Trust - I think they sometimes come up river on the Thames ?
Not sure exactly what you mean by the film has to be experimental, but if you want to get out on the water, I would guess a voluntary group would be most likely to help you out for free !