Hello Klanky
Since the boat is in the water, and you can't look at the outside of the keel very easily [:D], rather than trying to second guess external causes such as weed, or worse-barnacles, why not start by getting the owner to agree to unscrewing the table top?
It's not a difficult job, and then you can at least have a look at the lifting gear, the inside of the keel casing and the top of the keel itself.
If all seems well internally, then, if it were me, I'd ask the owner to let you see the boat in slings with the keel down...
Good luck!
Geoff Sheddick
Parker 27/146 "Stroller'
Geoff Sheddick
Parker 27/146 "Stroller'