I would feed straight salt if I wanted them to drink more. I feed electrolytes if they are hard working and sweating a lot. My horse is a huge sweater, so he gets electrolytes May through October (or a little later) when he is sweating nearly every day to replace what he loses. I give him an extra helping on the really hard days (very, very hot days, xc days, gallop days).
Love the Smartlyte pellets. He's not a fan of the powdered electrolytes and will not eat very well. These he eats great, so he gets both his electrolytes and his much needed calories!