Silver: Moka's rosary is made of silver...so you can say that silver negates their strong auras, making them relatively weak. Of course, this is just my take on it.
As for Moka's water problem, she's anemic so you can say that she rarely gets 'enough' fluids in her diet. She's had fainting spells before (being dizzy and so forth). So with Tsukune here, she's in much better shape but she hasn't gorged herself on anyone or anything but even so, she has so much power. Tsukune's blood was pretty potent from before, being a virgin and all.