Re: general fitness questione...
Are you always off balance? Walking, etc? Or is this just a skiing/wakeboarding issue? If the former, see a doctor...possible inner ear or other health issue that needs help.
BUT, assuming it's just watersports where you feel off balance, then I'd suspect technique is to blame, rather than anything physically wrong that you think needs to be improved on.
Those who have balance problems in watersports are usually the ones who are looking down...looking at the wake they're about to cross, looking at their spray, looking at their ski/board, looking at the rough water and wondering if they're going to get through it.
Never never never look at the ski or board, or at the water you're about to hit. Just keep your knees a bit bent and loose so they just absorb whatever happens to come, no matter what size or whether you see it or not. Always look at the boat, or at the horizon, or at the buoy you're aiming for, or something OUT there. But never DOWN. Keep that head up!
FWIW, this is gymnastics 101...try walking a balance beam sometime if you have something nearby at a playground or something, or even just a curb on the edge of the street. If you look ahead and keep your head up while walking it, you can confidentally walk the beam without thinking about the balance. But if you stare at your feet and concentrate, I guarantee you'll tip over or get real wobbly.
Also...make sure your handle is down closer to your hips, and not straight out from your shoulders. I am admittedly very new to wakeboarding, but this tip helped me tremendously when I was learning....and I got past the wobbles REAL quick when I started focusing on this. Having that handle lower will lower your center of gravity and helps tremendously in many ways, making turns much easier and more precise. Incidentally, this makes more sense and feels more natural when you're turning aggressively and cutting hard, whether on a wakeboard or even on a slalom ski. If you're just following the boat in more of a resting position and moving gently/slowly back and forth over the wakes because you're not real confident yet, then this technique can feel a bit odd and unnatural. But try it anyway. I'm not sure what you're skill level is to be able to comment much more.
But if you focus on those two things...especially the looking down thing, I bet you'll see some improvement.
