“A distinction must be drawn between boats which may stop temporarily in the course of navigating along the river, and those which are not just, as it were, passing through. In relation to the latter, the unauthorised attachment of a boat by mooring to the Claimant’s land will constitute a trespass, as will a material stationing of the boat, even if not by physical mooring to the land (such as by mooring to a post standing on the river bed).”