I believe the aptly named Hell’s Gate is the limit for navigation.It is not far upriver on the Fraser Canyon, just downstream from Boston Bar.It was named by early explorer Simon Fraser, who wrote that navigation downstream through the narrow, 115-foot passage was virtually impossible.
The OP failed to define the type of boat.An ocean going vessel is realistically limited to New Westminster.150 years ago the smaller vessels docked at Ft Langley and might have been able to make it a tad farther.
Steamboats designed for rivers could make it past Hope to Yale where rapids and shallow water prevented any further travel.
Practically speaking, not much further than Hope.The Fraser Canyon, is navigable a bit past Hope, but going up stream would be very difficult.
The town of Yale is considered to be the head of navigation with a riverboat.Just past that point the river turns into rapids.In the days of the gold rush Yale was where the boats stopped.
You can see the Yale townsite is the lower left corner.In the upper right you can see it turns into rapids past Yale 20.Even where it says Yale 18 is rough water.