Brilliant! Thank you very much. I knew I'd read something somewhere.
Someone reminiscing in the Newbury Weekly News in 1878 wrote "A couple of Mr. John Flint’s, or Mr. Evans senior’s, or the late Daniel New’s ‘wussers’, strung together as close as the Siamese Twins, are returning perhaps with loads of the much-prized Newcastle coals that had been ‘whipped’ from the northern coal-hulks in the port of London..." This sounds like wussers were narrowboats, which fits with your two quotes.
Then I found another reference which calls a Kennet barge a wusser. But journalists are not always accurate...!