Re: surf and gym classes
My philosophy is that without underlying strength, you will never get good performance (in any sport). You are also more prone to injury. Unless you already have a solid strength background, I would concentrate on that. Yes, my advice contradicts Billie's...
I would go 2X strength, 1X pilates (which also assists strength) and maybe 1X boxing if you want (or any of the 'cardio' classes, so long as they are 'interval' training and not just 1hr of low effort cardio). If that is too much, I would drop the pilates if you are young (the strength workout will give you a good core workout and you are probably still flexible). If you are older its a bit more difficult because pilates will be useful - maybe alternate weeks pilates and boxing.
You will be surprised at how cardio effective a good strength workout is (by which I mean short - 30 to 60sec - rest periods). You can easily have a high heart rate for an hour. Plus its good for fat loss...(better than running)
Obviously none of the classes are specific to surfing, but for most people they will get far more benefit out of improving general strength levels (and cardio ability) than trying to be too sports specific.
That said, I'm not sure what a 'strength class' involves. If its not a good quality lifting heavy-ish bits of iron multiple times activity, but rather some kind of cardio workout with weights, then its really just a cardio class like most of the others. In which case, just mix up the cardio classes and find a proper gym!