A couple of ideas for water filling:
1. When doing a whole course worth, use a weed sprayer with an electric demand pump--like one you set on back of an ATV with battery terminal clips. Make your needle assembly by putting a basketball needle into an air nozzle (like you would use with an air compressor to blow dust around your shop). Get a male thread brass barb fitting and hose clamp into the sprayer line. Screw a female air chuck onto that to hook your nozzle/needle to. You can half-fill each ball in seconds with this. Keep a second air nozzle with just a rubber tip (no needle) on it for filling the boat guides with air or water if you use the ling ones that have a bigger port/no needle orifice. These go really fast like that.
2. For maintenance and replacements, boatside, get a 1 or 2 gallon pump sprayer-- the kind with a hose and wand. Cut the wand off the hose and clamp in the barb/chuck/nozzle fitting per above. Dip your sprayer in the lake to get your water, pump it up, and fill a ball in little more time than with the electric pump. Then, to finish filling with air, just turn your sprayer upside down so the internal dip tube is out of the water and all you get is air. You can also use without water for fast air filling, if that's all you need. That way, us public-water skiers can just keep a box of flat balls in the boat so the ski locker isn't jammed with enough inflated balls to get us through a weekend of wallies.
Can't post a picture of my setup today, as it's in the trunk of the 'Bu in the slip but I'll try to remember to do it this weekend.