Kacper...SailingFool is absolutely right. Do not SAY or put in WRITING that the boat is not worth $12000 based on what your surveyor said. You already have a signed contract saying that it IS worth that much. You can (under most broker contracts) walk away if the survey is not to your satisfaction but you CAN'T walk away if you suddenly decide your offer was too high. Stating this may jeopordize your 10% deposit.
By all means...counter offer based on the SURVEY FINDINGS OF FAULT or Walk Away...but don't dig yourself a hole! Good luck!!