Hi Stephen, we have a Dufour 365 GL which we have owned since new in 2010. Unfortunately we are selling it and are currently away so cannot compare your photos but here are my comments.
Firstly, there are holes into the sump from other areas of the hull to allow drainage to the lowest point. For example draining the calorifier each season water goes into one area and gradually dribbles into the sump. Similarly you do get condensation needing to drain down and any other water getting inside. Hence, in my view the holes.
I have never tried poking the bottom of the sump to see if there is a hole at the bottom towards the actual keel but that seems unlikely. Why would they put one there?
You say the water is salty but how do they know, and how salty?
We often find a little water in the sump, mainly condensation I suspect. I have never tasted it to check.
Of course you need to take into account what the surveyor says but taking the keel off when there appears not reason for any damage (unless there has been a grounding and damage to the keel itself repaired) to have been done seems drastic to me.
Have you spoken to the owner via the agent and asked for any declarations? Was it a charter boat, maybe more likely to have had damage?
Good luck!
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