From your first post, it seems likely that the problem could be from back siphoning.
If the through hull
for the bilge pump
is under water
it should have a valve on it. If it doesn't, you need to either put one on it, or take it out and seal the hole.
In my opinion a bilge
outlet should never be below the at rest waterline anyway.
And if the outlet is only a few inches below the waterline you can reposition the through-hull and seal the hole, change the fitting to a seacock, or add a valve
to the existing through-hull (in my personal descending order of preference), by, on a calm day, making the boat stationary (via strategically place anchors or dock) and using a halyard
from the masthead to the appropriate beam position to induce enough list to pull the offending through-hull clear of the water
It's a little extra work, but it beats paying 500.00 to have the boat pulled, and of course the location of the through-hull could put paid to the scheme anyway (but probably not).
And that doesn't mean the leak will be fixed, but only that a potential problem is averted. But you may get lucky and kill two birds with one stone.