On the Southern Broads you could sit outside The Nelson at Reedham, or Reedham Ferry and be by the water. Not really gardens, but patios. The Ship Inn at Reedham is more of a garden but quite close to the swing bridge and the view is quite limited, especially if boats are moored alongside.
The Bell at St Olaves has a nice riverside garden, but the boat traffic is more limited due to the low road bridge.
Probably the better locations would be The Coldham Hall Tavern Brundall or The Ferry at Surlingham. Both have nice gardens and good long views up or down the river.
Further up river you have The Waters Edge Bramerton with a nice outside decking area and good views of the river.
If your looking for good plentiful pub grub, then The Ferry Surlingham would probably meet all your requirements the best.