The advantage as a cash buyer is no financing contingency in a contract. As a seller, I'd consider that a modest plus. But I think it goes further than that because a financing contingency may well turn into a bunch of survey issues that must be fixed to get financing.
So as a seller, I would give some favor to a cash buyer. How much frankly depends on other factors, like how much interest there is in the boat in the first place, how flexible I am on price, how much risk I think there is in getting hung up over financing/survey issues, etc. Also the older a boat is, the more demanding and discriminating a lender will be. If I were selling an older boat that might be hard to finance, I would put much more value on a buyer without a financing contingency, because it might be hard to finance the boat no matter what shape it's in.
As a seller, the big risk in entering into a contract is that you lock out other buyers while the contract is in play. So the more likely I think a deal is to go through, the more likely I am to accept an offer. The OP is looking at newer, quality boats, and those will be the easiest to finance. So in this case I don't think being a cash buyer is much of an advantage. But hey, play every card you've got.