I think that you really need to take a step back from the boat thing and look at what it has made you think about your own finances. It seems as though you are not sure either on your own or with your husband exactly where you are and how you see the future for yourselves, so this may give you a very good opportunity to discuss your own finances. If you can do this without making it a connection to the boat issue, hopefully you can actually get together and just work our both your present and possible future financial situation. Then you will have a clearer view of your own situation and you may be surprised, either pleasantly or otherwise about how things stand.
So if you overall see that the two of you are in a good situation then you may feel better about giving this money to the son, because I think you also need to ensure that your husband does not live in cloud cuckoo land and realistically sees that it is a gift not a loan. Then this may lead on to discussion as to whether this is actually in the best interest of the son. If this is going to be a one off amount of money that your husband can afford to give him, would it not be better to put it into something safer with some interest that he might use, or something like that. ?
We have had boats of all sorts from canal boats to old sailing yachts and I know that we had a lot of pleasure from particularly the sailing yacht and also sailed up to scotland which cost us only £8 for the three of us as we only put the engine on to go into a harbour. But there are definitely warning bells from the houseboat, I could see it being an ever open pit for money. Perhaps the best thing would be to get a qualified boat yard person to go over it and give you a realistic idea of the costs of repairing it and whether it would be worth the time and the money, before you spend any more money. If you pay for this ,and make it clear that it will not be a job coming to their boatyard, you should get a realistic idea of both cost and time scale and then have a clearer idea of whether this is even a possible idea. Also by getting a knowledgable person to do the vetting of the boat, you are keeping out of it and their opinions will be clearly business like and there will be no sense of personal thoughts altering or affecting the outcome. You could either just ask someone appropriate to check it over and give you a written report, or you could all be there and hear what they have to say , so that once again having the independant person there you are not pushed into a situation where your common sense approach is seen as deliberately against the sons idea.
Hopefully, from a situation which seems fraught with problems for you all, you could actually end up in a better place, where you have a clearer view of your own situation, you have shown respect for the sons idea without committing yourselves to the costs, and although it sounds rather pessamistic, I think that besides the concrete knowledge of future costs for your husband and yourself, then you need to have some financial backup for some unknown situation that might arise in the future for y ou both. It is clear that you could not depend upon his son to be able to be helpful to you in the future or to rely on him in any way. One other possibility is that you might look at your own situation, and for example if you think that in the future ou would leave X amount to him, you could allow if the money was available, for a certain amount to be given now, but on the clear understanding that this means that he will not have this money to come to him later.
Perhaps, before you do any of this, you have a close respected friend or trustworthy person who you could go through this with , so that you have can check your own feelings out and follow through how you think things will change depending on what you decide to do.You need to see all the possibilities so that you have thought about how you would feel if whatever you do, means that there are either serious arguments between you and your husband or it means that there is some sort of emotional blackmail by the son etc. This will allow you to think what you might do in any specific way. Then you should be able to talk to your husband in a calm way as you will have looked at the various possibilities. Do hope that at the least you both go forward knowing more about not only your financial situation but being closer to each other. Best of luck.