I thought diverting the Ouse west to Falmer then back down to Newhaven would solve it, but what about flooding the the redundant railway line and using that as a canal perhaps?
What about getting planning permission and doing engineering work to divert the Ouse westward from Lewes down to Falmer, then back out to Newhaven, then implementing a high speed, shuttle, hovercraft service between Shoreham, Falmer, Brighton Marina, Newhaven Harbour and Sovereign Harbour.