Anyone else experienced anything like this?

[MermainQueen]

We recently went to check on our Sun 2000 daysailer at its storage location on a farm near our mooring (its been on land about 3 years due to life not being convenient for sailing) and were horrified to find around 18 inches depth of freshwater in the cabin. It seems that whilst in storage rainwater has been leaking through the companionway hatch and slowly filling the boat. We have drained the majority out with a combination of bilge pumps and sponges and removed all the interior wood and cushions which all need to be replaced. On assessing the damage we have established that there is water between the liner and the hull which we can't get at which has got there via the holes drilled in the liner for the sink plumbing line. We have no way currently of working out how much water remains in this space and no way to access it to drain it. There is osmotic blistering evident to the grp liner although it doesn't look too bad. We have no idea if there is blistering to the hull. Has anyone experienced anything similar? Any ideas re how to get the remaining water out without causing more issues? Do people think the boat is salvageable or will it be a write off?? Any ideas how to assess how much water remains?? We think the water has been building slowly over 3 years but likely only overflowed into the liner/hull void in the last year judging by the height of the plumbing hole. Thanks in advance.