You asked, so here's why I pre-pay for water:
I am admittedly horrible about remembering to drink water. Misery ensues, especially when hot weather, sparkling wine, margaritas, and gin & tonics enter the chat. Yes, I can fill at the tap or the buffet (ignoring the signs that explicitly say not to) and I can and do bring my hydroflask for that purpose. BUT...when I pre-pay water, I "assign" myself a certain number to drink every day. I throw them in my bag when going ashore and save a trip to the buffet or the table as you walk off the ship to get one. I drink half a cold one from the fridge when I get up, and the other half before I go to bed (I can't stand room temp water). Again this is just me, and how I choose to travel, and it works for me because being dehydrated and hungover on a ship is the worst thing ever. Having the cartons ready to go in my stateroom when I arrive is just my insurance policy. At home I drink 64oz of water a day. On a 7 day cruise I drink 24 liters of water and it's $50 well spent, at least to me. Your mileage may vary. To each their own. Cheers