I lasted around an hour (I forgot to start toosmartwatch) and I reckon (conservatively) I covered around 2.2k.
It was another glorious evening and I swapped the Godfrey's for Bob's Aquaseals which dramatically helped with the navigation. After a few little circuits around a couple of buoys off the slipway, we struck out for the start of the rowing course around about 800m upstream (allegedly).
You can see the effect of the current on the way up which we covered in two stages, and coming back in one longish swim.
Total 2,200m (or thereabouts)