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Here is an extract from my website under boating tips and chosing a boat. This information is refering to a holiday cruiser not a day boat.
"I do not believe that any holiday hire boats can accommodate onboard wheelchair use nor provide easy/wide access but Barnes Brinkcraft do have a boat in their Brinks Braemore Class that provides wheelchair access via a stern ramp. They suggest you ring them for details. Tel: 01603 782625."
It depends what you want. If you would rather have a driver and on board facilities then I would suggest the Nancy Oldfield trust. The drivers are all RYA qualified and volunteer with the trust. If you call the office on 01692 630572 then they will sort you out - They can offer morning cruises, afternoon cruises and all their motor boats are equipped with disabled access toilets and wheel chair lifts. You can design your trip to be short / long and there are a choice of routes you can take.
If you want just a day boat then Ferry Marina in Horning have a number for hire which have wheel chair lifts as do Norfolk Broads Direct in Wroxham.
Try the trust - I know you will have a brilliant day.