Thanks everyone, for your honest opinions, and good ideas
First of all I want to clarify that I'm not planning this as a "party", and I would not present it as such. DH and I are going to go on the dinner cruise regardless of whether anyone joins us or not. As of now, I do not plan on telling the cruise that we are there celebrating a birthday at all, in fact.
Secondly, I definitely would tell the people I want to "invite" exactly what I've said here; that I have this surprise planned for DH, that friends of ours expressed interest in joining us, and basically just give the information, including the website for the Spirit of Boston and price of the dinner cruise, let them know they are welcome to join us, and leave it up to them. I do know that if I didn't mention this to a few of the couples, afterwards if they found out that the reason was because I couldn't afford to pay for everyone, they'd feel hurt at not being given the chance to attend and pay for themselves.
I would be inviting 3 couples who are friends, DHs father and stepmother, DHs brother and SIL, and DHs sister and BIL. None of the friends are friends with each other, although they have seen one another at get togethers at our house before, so I can't use the idea of having one of the friends pass the word along. (although it was a good idea!) DH's sister and brother live in different states and really don't keep in touch much. So it would be up to me to contact each couple, one way or the other. I think a phone call would be better than an email though.
P.S. I definitely would pay for at least one round of drinks, maybe more.
The more I thought about it this evening, and coming home now and reading the opinions here, I think I will give it a shot. Those who know me well already, know I am not a tacky person, and those who think I'm tacky just don't know me well.
P.P.S. Eros: I understand how you're viewing this. I don't even like to have people bring food when they come to a party at my house even when they offer, I like to supply everything and invite people simply because I enjoy their company. I do feel this is a different situation, though. But thanks for your opinion. Perhaps someone I "invite" will feel the same way, but I don't want to leave out people who wouldn't think twice about joining us (at their own expense), either.
Now if only I knew in advance of who might feel this is tacky, I just wouldn't mention it to them.
