I know a bit about this, but this only covers a small segment of the market, but it is probably not too far off...
Typically around 10% in the US and 15% in Europe
, but it varies in that region for the basic boat. Optional extras give a bit more, depending on whether they come from the factory or are added locally.
For this amount they also have to handle a certain level of warranty work, so dealers prefer to sell good quality boats.
Demo boats are often sold to dealers at around 5-10% lower prices, to encourage having a real product to show.
Some dealers only work on a commission basis, which means lower margins.