That is a mixture of diesel and water. It all needs to come out and be disposed of. What Bizzard says is right, you or they need to check, maybe endoscope?, and get rid of the sludge and rust in the tank, If its at all possible it would be good to pressure test the tank to make sure it has not rusted through somewhere. You are not alone, I scrapped a large amount of expensive road diesel a couple of years ago in my boat and it really was a right pain. It shouldn't damage the engine as it simply will not run on that stuff . Mention all of this to your surveyor and trust him to check the tank and the engine. It doesn't write the boat off, there are lots of boats with horrible things in their fuel tanks but their owners don't know (yet)