I'm 31 so not as old as some of guys posting here but I've had a lot of injuries in my life so my body feels way older than that. I often find myself lifting and doing things like a 50+ year old man. Most of the things people complain about I've dealt with and most of them I've overcome so I'll share what has worked for me.
1: Finger numbing or cramping: A lot of this is actually due to your finger throttle position. We all know the handlepole moves but you're in one standard riding position prior to doing a trick or looking for a wave and this is the position you need to tailor your throttle to. Make it so your wrist is in line with your arm and your index finger should be relaxed and slightly curved down on the throttle lever. Your finger needs to be pointing slightly below the same line as your wrist and arm. If your typical glove size is a medium or smaller, you need to be running ruffian grips and not the big thick rogues. Too thick of a grip will kill your hands.
2: Back pain: This one is a little harder to overcome because it requires some modifications to the ski and/or riding style. If your back is on fire after riding, your pole is too short. Your stance and foot position in the tray has a bigger impact than you leaning forward at the waist so don't compensate by shortening the pole if the ski is riding nose high. Move your feet up, ride with the pole at your waist, and leave your entire body forward instead of bending at the waist. Think about leaning into a strong wind and allowing it to hold your body angled to the ground.
3: Blisters: Quit buying mountain bike gloves or jetski gloves which seem to be the same thing as winter mountain bike gloves. Get yourself some nice kevlar waterskiing gloves or at bare minimum some gloves made of Amara. They are more expensive but the extra grip will amaze you and they will last for years instead of half the season. You'll probably have to order them from overtons or another boating store like that because I've never found a nice ski glove at a local store. Most importantly....Waterskiing gloves do NOT fit when they are dry so if they do then they are too big!! I've been waterskiing and jetskiing for 15 years. I just bought my 3rd pair of gloves.
4: Sinuses: I haven't found a good fix for this. If I get water up my nose at any point, I will be wiped out for days. I usually feel like I've got a cold with a runny, stuffy nose, scratchy throat, watery eyes, and absolutely no energy. I've tried nasal sprays, saline flushings, etc etc and none of it works. I used a nose clip last year some and it makes the following days better but also makes the riding less enjoyable so it's a tradeoff.
5: Legs: If you aren't running 1" padding or more, you need to be and don't scallop it out under your feet. Run the full thickness. My legs still hurt some but no where near as much as when I ran less padding or cheap padding.