You are correct, they don't list all the tastings on the navigator. My suggestion is book as many beverage seminars in advance as you can after you do your online check in. If you can't get them immediately go to La Piazza once you board the ship to see if anything is still available. If you don't have any luck, check with guest services and/or La Piazza each day for openings. Also, I have never had any luck, but I have heard if you show up for the tasting if there is an opening they might let you buy a last minute ticket.
As a previous poster said - get to know the bartenders. On the Fantasy I can highly recommend Andrae from Jamaica - and I know a few others on the boards will recommend him too. On our last cruise he was at La Piazza.
There are also drink "cards" you can get - Skyline has the drink passport - you pay a flat fee and get to pick drinks from their specialty menu. It equates to buy 5 get 1 free or buy 6 get one free, it's been a couple trips since I did it, but you get a passport to bring home. La Piazza also has a map that you buy and you visit the other bars that are on that map (La Piazza - Italy; Oh La La - France, O'Gill's.... you get the idea) I only learned about the map at the end of our last cruise so I don't know the cost or what drinks were offered. We always just ask Andrae to make us something "different". I've always said I don't like bourbon - but he made me a bourbon drink that was very good. I hate Campari - but he made me a drink with it that I asked for multiple times.
As for tastings we have done several - I can HIGHLY recommend Mixology, each one I have done has has different drinks. I've also done tequila tasting, martini (not a fan, but the martini "style" drinks were good), whisky, bourbon, and beer all on the Fantasy over the years (we've done beer, wine and tequila on the Magic as well). If you like bourbon or whiskey they are a great way to taste different brands and on our 2 bourbon tastings we got different bourbons. My friend did the wine tasting and enjoyed it. The one I have not done and will be booking on our next cruise is the liquor and chocolate.
Have a fabulous cruise!