That is the typical rate. You can negotiate less sometimes, particularly for an expensive boat, but you need to be careful.
Just like real estate, the total commission is split with the selling broker, usually 50/50. If you negotiate less, say 8%, then the selling broker may get less thereby reducing his incentive to show your boat. Always work out your agreement with the listing broker so that the selling broker gets the full 5% he expects. In other words, take the discount out of the listing broker's part of the commission.
In some markets, listing brokers sell 90% of their own listings, so it may not matter. But discuss it with your listing broker.