I know the trope! I'll claim that bonus.
Uh, probably because humans (and other animals) salivate - you're tasting water almost all the time, so you're used to it and it tastes neutral to you. I'd imagine that's also why water tastes so refreshing when you're dehydrated and have less moisture in your mouth; 'refreshing' is the default flavour of water, but you don't notice it under normal circumstances.
Inklings can't drink water - how about juice?