Re: Seller agreed to offer, but set a clause,"no discounts after survey"
That would be fine for any obvious defects - what you see is what you buy.
It is not acceptable for latent defects. If the survey reveals recommendations that you could not have reasonably discovered by your thorough pre-contract inspection, then you and the seller should be willing to re-negotiate, especially if you have reason to believe the seller must have known about the defects.
If you are using the standard broker's contract as some members advise, you have very little recourse in any event, since the contract is largely conditional until after the survey takes place (and you have already spent hundreds or thousands of dollars, thus putting you over the proverbial barrell). In this buyer's market, I have advocated using a buyer's contract that requires the seller to reveal known latent defects, among other things.
Change the clause to discover the seller's willingness to allow re-negotiations for recommendations concerning latent defects revealed by the marine survey, or walk. Othewise you are out all the costs to survey the boat with no expectation of any re-negotiation.
An honest, reasonable seller should be willing to agree to that because he or she would be morally (and perhaps legally) obliged to reveal the latent physical defect to the next buyer (or face a fraud-in-the-inducement claim for the buyer's survey costs and expenses - depending on the contract used).