Lower Thames Rowing Club are holding their Southend Pier race on Sunday Aug 25. It is a seven mile race from Leigh-on-Sea to the pier and back. Details are here: http://www.lowerthamesrowingclub.com/lower_thames_rowing_club_pier_race.htm
The race is only two weeks before the Great River Race so crews will undoubtedly be training, but it could be a useful warmup for the main event.
If any crews want to participate, they should get in contact with the organisers themselves.