are the fat sacs just filled with water?
i'm 99% sure yes, i've only actually seen one in person (and that was on a piper,not a boat) and i remember the guy had a little 12v pump that plugged into a ciggy lighter to fill, empty, it was basically the water equivalent to the little doo hickey's that fill air into your tube's so you can add/subtract on the fly.
i remember looking into pricing and said,,, "holy crap !, they're like $150 !!!"
anyway, alot depends on how u wake and how hard, we used to move ballast from port to the goofy side when we were practicing certain tricks left or right, but most of the time it was center/rear and port heavy, no many of us were goofy side., so in general we did center ballast and the port side so the bigger wake was on the left side, smaller wake on the right so riders could go big or go small, we didnt' have any right side riders so we never needed to reverse the ballast, just took some out for beginners and added to the port side for advanced.
all this is the reason why portable ballast is necessary.
before we started using sand packs we we using water jugs, anti freeze jugs, hex weights, anything thats small and heavy and could be either strapped in, or wedged in between a couple of things so they wouldn't be sliding around.
imo, go tubesand, its so much cheaper but if you have money to burn, get 3-4 fat sacks, and a tsunami pump, and your good to go....
weight the center, port, rear first, do a couple of runs, and try to experiment with different setups, slide out of the port and put more rear, take out of center and slide toport, etc, and you'll find one that fits well for most of your riding,
oh, don't forget to watch your weight, adding ballast, then 3-4 passengers and your at max load pretty quick, other riders make great ballast and as long as you don't need to feed them, they come cheap !