Be proactive. Call him at whatever interval you want updates. Ask him for details on the "someone" interested in your boat. He most likely won't give you their contact info but he should be able to tell you details about the buyer: young, old, couple, individual, experienced sailor, first time buyer, what did they like about your boat, what did they not like, etc. You should be able to tell fairly quickly if he's actually showing your boat or just pretending to.
If you can't locate your ad on your personal computer, tablet or phone
and neither can your friends then inform the broker and tell him you want it fixed yesterday. Make a note of how long it takes to resolve the situation. Poor performance in the marketing
of your boat is, IMO, ample reason to terminate your contract with them. But you have to document the poor performance in order to protect your right to find another broker or to sell the boat on your own. Your contract will have specific language protecting your current
broker as long as he is upholding his end of the deal. To void the contract you have to show he is NOT following the terms of the contract.
Unless there is specific language in the contract allowing him to change the price then this alone could constitute breach of contract. Contracts that I have seen specifically state any price changes must be approved by the seller in writing.
Your requests are legitimate. The broker is constrained only by what the contract requires. If you have a mediocre broker he may only do the minimum. Express your concerns to the broker in writing. If there is no improvement in performance then take steps to cancel the contract.
I'd be willing to bet that if you make enough noise
you will either get better performance or release from the contract.