The first thing I would try, especially if the boat has been sailed a lot in salt water is, put the boat flat on the ground (grass) so that the hulls are supported and then using a fresh water hose, push it into the hulls and fill them up. Salt accumulates in hulls and isn't so easy to get out unless you actually try. About 10 years ago someone in the UK did the cut remedy on an older boat and found ~ 1 years salt supply for Muck Donalds in his hulls...
Try rocking the hulls to get the water to wash any salt or sand to the back of the boat and drain it out the bung hole at the back.
You can also check by undoing the hatch by the front cross beam and putting a hand in there to check.
Also check if the boat has been patched... that could account for it.