The accepted "normal" deposit is 10% of your offer. Most of this is perception, if you want the yacht you need to display your sincerest intent. Staying with the normal 10% shows the seller that you aren't just fishing
Remember to read the contract
word for word and don't be afraid to get explanation of paragraphs like "seller breach" or "buyer breach," disposition of the deposit etc.
Just because you write an offer does not mean that you have an "option" to hold the yacht off the market. The seller has rights according to the contract
read his/her rights. Many contracts allow the seller to accept a "better" offer if the seller returns your deposit and pays any expenses.
If you offer the seller a "counter offer" to renegotiate the deal for any reason you have let him/her out of their contractural obligations, he could refuse your counter offer and take the next buyer's offer by just letting the counter expire.
I am retired now but have 40 years selling yachts, reading and negotiating contracts, I have owned brokerages in Florida
Glad to be of any help I can, keep the process fun this should be a fun time in your life. If it doesn't feel right back out and go to the next boat. I can guarantee there are more boats.