Changing water pump with the cambelt is the norm on many vehicles because its an easy cheap job.
But on the Ducato its a 'jobus bastardus', very easy to snap a rusted in bolt which will be an engine out job to replace.
Long running thread on the OaL forum where a guy took his Ducato in to a garage for a service with cambelt change ... snapped bolt trying to replace the water pump .. engine out .. tried to drill the snapped bolt out, drill slipped and ruined the block ...weeks of delay ... cost running into many thousands .. interminable arguments over who should pay for it all.
Nick Fisher who ran Euroserve in Leicester leasing out a fleet of Ducatos posted they had never changed a Ducato water pump (except once when the engine was already out), never had a water pump fail, and most of their Ducatos do around 350,000 miles on lease hire before they are moved on. But he recommended replacing the cambelt tensioners - said they always looked worn