Unfortunately, you're probably right. It would be torture. I'm about 50-50 when it comes to getting seasick, not as bad as some, but I've definitely had my spells. I managed the crossings fine on my last trip (August a few years ago) because I properly prepared myself (Dramamine and lots of wine), but got nailed hard when we hit some unexpected nasty conditions back in the central islands. That was also when our toilet exploded and I was so seasick I couldn't get out of bed to escape.
On the other hand, the diving was so awesome at Wolf and Darwin that is was worth every minute of the torture. Tell your husband to quit whining, be a man, and deal with it. It's only 12 hours or so. Women handle the agony of labor and childbirth, cancer victims suffer endless nausea from chemo, surely your hubbie can tough it out a few hours so you can see whale sharks?