Where did the risk come from?
In Tomlinson the House of Lords held that an ordinary stretch of open water presents no danger due to the state of the premises: the lake was no more dangerous than any other body of water, and nothing about its physical features caused the injury .
The risk instead arose from what the claimant chose to do on the premises. Lord Hoffmann thought it would be extremely rare for an occupier to owe a duty to prevent people taking risks inherent in activities they freely choose to undertake .
What cuts against you
The pontoon is a man-made structure the occupier installed; the claimant will argue it is a feature of the premises that invites the very activity complained of, which distinguishes it from the natural lake in Tomlinson. That is the argument to prepare for.