If you keep your home on the cool side, I'd imagine that would deter them from jumping into the water. If they're warm, they'll welcome the relief of a bath. I know Chippy will bathe quicker when he's on the warm side.
He likes kale, which I like to use because it's a tougher lettuce and won't wilt so quickly. Broccoli florets are usually a favorite starter veggie. Just keep them fresh and even in water so they stay crisper.
I think they will try new things when they feel a bonding with you and a trust to try thing you offer.