You obviously cant see the problems with your plan....
What if you DO find another prospective buyer who would be willing to go in with you on a haulout/survey, and it so happens that you both decide you want the boat?
at that point you cant trust the other buyer and he wont trust you... you have just lost a lot of your negotiating power because now there ARE two buyers wanting it... and how are you gonna decide who walks away?..and if you do the gentlemanly thing and bow out, how are you going to feel knowing that you just paid half of the haulout/survey costs to help the other guy buy a boat that you wanted?... or maybe you think he will bow out and let you have it?..
How are you going to locate the other prospect buyers?... the seller wont tell you because he doesnt want prospect buyers getting together and attempting to conspire against him.
its just a bad idea all the way around. The ONLY good that could come from it, is that neither of you want the boat and you both walk away from it, having each only paid half as much for the knowledge.
BUT, moving forward on the premise of this theory, it brings up another problem/question.... when one is ready to have a survey done, they are reasonably certain that the boat will be found to be in buyable/affordable condition. it not always is, but the initial feeling is there.
from all your thoughts in trying to get someone else to pay for part of the "discovery" costs, it sounds as if you doubt the boat will meet your demands. (or you doubt yourself in being able to choose one in reasonable condition) If thats the case, you should walk away from that boat now, and find one that you are more excited about...