By hooked to the riser I meant the hose was attached to the exhaust riser. I always pull water from a bucket.
If you "uncloak" and fill in your boat model and year and sailing venue, this can sometimes allow more focused help.
For instance, I have an '88 M25XP. At about 1900 hours we replaced the entire Oberdorfer raw water pump -- not because the impeller was bad, but due to wear in the cast pump chamber. In our particular river there is significant silt present year around in the water and it is actually a very fine but "sharp" sand of volcanic origin.
Previously we once had to machine the grooves out of the cover plate, about a decade ago, and found that in the last year even a new impeller would not move water as fast as expected.
With the wearing away of the metal, the pump chamber gets a bit larger inside and the rubber vanes start losing some of their sealing and pumping ability.
Perhaps not a problem for you, but an example of why some parts (sometimes) do need to be replaced.
So, how may hours on yours? Water siltation a problem?