Does “watering” a plant pokemon involve more than simply providing enough drinking water?

It could. Usually, giving them enough drinking water in a dish is enough to keep them hydrated, but some—such as bellsprout and oddish—prefer to be rooted in dirt and watered as you would a houseplant, either instead of or in addition to providing water in a dish. Likewise, while the plant half of some pokémon (such as members of the bubasaur line) remain hydrated simply by sharing fluids with their hosts, it’s often more effective to water them as well.