Listen man, you are not in as deep a pile of doo-doo as you think, and here's why: you know and are willing to acknowledge that there are issues outside of the boat that need to be worked out. Here are a few things to consider about this boat and your wife:
1. Unless it was her idea to buy the boat, this is your dream and not hers. Buying a boat does not automatically get her to buy into The Dream. They are two completely separate things - and this is something that many of us (me included) get confused about. The time to get her buy-in is wayyyy before you start talking about boats.
2. She doesn't trust the boat. You said "problems" which can mean a wide variety of things. To be honest with you, if I couldn't swim, I wouldn't want to go out on the sturdiest and most capable craft.
3. She doesn't trust you to captain the boat safely. You hinted that you might know why, but I doubt it's as simple as your bro got the dough or your business never took off.
Here's the solution: work on your relationship before your boat. In fact, if you love your wife, forget about sailing this boat for a while. Show the kids how it works and give them the joy of sailing. You might even consider just giving them the boat as long as those "problems" aren't too serious. Go out with them every once in a while for a smile, and let your wife wave from shore and join for supper afterward. Whatever you do, take the pressure OFF of her completely for now. The pain of lost money is only once, the pain of this boat just keeps going and going and going...
Now, to have greater influence over your wife and her happiness, you need to be more likable. To do this, you need to do the following:
1. Be there. Spend as much time around her as possible. Doesn't matter what you are doing.
2. Find common ground. Doesn't matter if it is peanut butter sandwiches or tennis or the children. Focus on what you share.
3. Use reciprocal affection. We tend to like those who like us. Tell yourself you're just getting the upper hand or whatever you have to do; fake it until you can make it but LIKE HER! She won't have any choice but to like you. I swear this is foolproof. Seriously.
4. Make her feel good. We like those who like us. So do it, yes, you know what I'm talking about, "honey, that sweater makes your eyes look great." Again, fake it until you can make it, but I'm telling you it will work.
5. Get her to help you do something. Anything. That's right, get her to do something for YOU. Her subconscious will tell her, "I must like this guy, otherwise I wouldn't be doing this."
6. Smile at your own mistakes. Be humble, man. Not saying you're not, but I am saying that it is the key to effective leadership.
7. Stay positive. We all like to be around happy, positive people. They have greater influence over us. So no matter how bad you think things have gotten between you two, start looking for the bright spots. They are there, so find them and focus on them. Be happy you have someone in your life at all. Many don't.
8. Develop your own strength and confidence. I'm not saying your not. I don't know you from any other anonymous post on the internet. But years of perceived failure in business and comparison with your brother would sure as heck shake my confidence if I were you. So how in the heck are you going to do this? Set some goals and attain them. Lose ten pounds. Run a 5K run or a 10K. Set a fun goal like you want to be able to carry your wife piggy-back for a mile.
Here's the big one: never give up. I mean never. I mean "no f'ing way I'm giving up." You are in your 50s? We met a couple on our first cruise who were just finishing an 18 year circumnavigation. He was almost 90!
The trick is to not skip over the obvious hurdles and don't pretend their not there. If you want your wife to GET ON BOARD, you need to work on the relationship first. That starts with you, not her.