I guess, if I had a husband like yours, I would present the idea the house will be worth more with air conditioning, will be more "modern" and sell for more money (unlike his boat which will lose value).

If you have a medical condition that makes air conditioning a good choice, I'd ask my doctor to write a "prescription" for one :-) I had a doctor that wrote I needed a heat lamp (for my shoulder ache) on a prescription pad and amused heck out of my husband. Maybe you can get a doctor's help and your husband will listen to him/her?