Having been involved in similar situations over the years I think you might be trying too hard. You might be surprised just how eager people are to come and speak with absolutely no commercial interest what-so-ever, just for the fun of sharing what they know. I have been involved in various clubs and have seen some really amazing speakers who have come in just to share what they have learned over the years. Occasionally the speakers do have an agenda, but it has usually been an agenda that the listeners were actually in agreement with. One example in boating would be to invite the coast guard to come do a safety briefing, for example, or demonstrate the use of flares, or whatever interests your membership. Invite long distance cruisers to come do power point presentations with pictures of their travels, or have a "show your boat" tour where your membership invites people over to see the improvements and modifications they have made to boats. The possibilities are endless. Invite an auctioneer in and have a fun auction of boat parts donated by local marine suppliers and donate the proceeds to charity. There are tons of fun things you could do, and they don't have to cost anybody anything. One club I belong to gives little paper weights with the clubs name on them to guest speakers - it's a trivial little thing, but it is at least something that shows the club's appreciation, I'm sure the speaker appreciates the applause much more than the paper weight.
I would shy away from anything that involves money, because it seems to me like that will quickly change the nature of the whole thing. Instead of it being interested membership listening to a committed speaker who shares a mutual interest, it has the potential to quickly turn into a disinterested membership who has to pay money to sit through some commercial junk they don't care about, and a disinterested marketing person standing there trying to sell product. If your membership wants that they can always go to a marine store on Saturday for free. As soon as you involve money, even if it's just a single 1$us bill for admission, then your membership will start to have some kind of expectation that they are going to get a return on their investment. It stops being "us" and starts being "you" and "them". Believe me, you don't want to be in that position, you don't want to be some guy who is being paid 5$us at the door to entertain everyone and make their attendance worth it. By keeping money out of it from the beginning you send a clear message that anyone who attends has to be part of the group and not just passive observers.
Edit - unless your yacht club really isn't a club at all but is instead a commercial money making enterprise with owners who profit, and members who pay fees to belong. If that's the case, then, well, you really are being paid to entertain people, so good luck with that ...