All the Navigators with underwater exhaust had the bypass installed, mainly for idle and low speed relief. It’s not a complicated system and worked well.
The curious thing is when one installation in a series has an issue when the rest don’t, It may be as simple as addressing the wedge in front of the outlet on the bottom of the boat, it was their to detach the water flow as it passes over the exit hole and allow the exhaust gasses to exit. It’s possible that it could have been left off or not built up tall enough. Another question would be does it happen on one side or both? Does the boat have Volvo 63L or 63P engines?
I recal one issue with a 61 Navigator operating in the Delta in NoCal. The shallow water was an issue, as an identical 61 was tested with the same engines and same underwater exhaust design and had no issues. The 61 in the Delta would live in skinny water for its owner and ended up being converted to side exhaust.And it was interesting to note that there was no issue observed on the 61 during its delivery trip from Newport Beach to the Deltaas it was in deep water, obviously. The Delta water depth was only 5 - 10 feet below the props at that particular location.
My experience is that the usual purchase surveys would not include exhaust back pressure measurements at a range of speeds as a standard package. This would be a special request at an additional charge, maybe to address a known issue .