We have been able to book onboard even though online the class appears to be booked.
@wacowgirl I think your chances of signing up on board would be good because it's just you. I'd recommend heading to the bar where they are handling reservations as soon as you board to see what's available.
We attended the whiskey tasting on our last cruise and it was not my favorite, but mostly because there were no mixers and it was all whiskey and it was at 10:30 p.m. If I loved whiskey, it would be a completely different story. I'd note that one person talked his way into the class on the day of the class and a few people didn't show up.
We've also done the tequila tasting which was awesome, but they did have mixers and I like tequila straight. Mojitos and Caipirinhas was similarly good.
Also on our last cruise on the Fantasy, we did mixology at Meridian and Damian was awesome! We made 5 drinks and it was really a lot of fun. While the drinks were strong, we didn't have problems with the amount of alcohol--the mixers helped and, because it's a cruise, our bellies had already been full for three days by the time we got to the class so we were well prepared for the intake of alcohol. Damian also gave us recipes for the drinks.
If your cruise should have a stop in San Juan, PR, I'd recommend the mixology class at Bacardi. That was a really amazing experience with a well appointed bar set up for each participant. We learned a lot about rum and proper mixology techniques.