Mel has made it clear that divers will swim the river in order to remove discarded fishing tackle and obstructions. Sensible and reasonable precaution. Most anglers who fish the Waveney do so with ledger weights or similar to hold their tackle to the bottom.
The problem comes when an angler retrieves his tackle to avoid a swimmer. Sounds contradictory but with the weight and a flying hook, or hooks, then there is a very real risk of hooking an angler as the tackle is retrieved. Most of us with boats have picked up anglers tackle and most of us who regularly fish the Broads have lost tackle to boats, it happens.
I agree, anglers can avoid the event, if they know about it. However if a match has been arranged for that weekend , and it might well have, then I doubt that it would be cancelled or relocated, indeed why should it be?