Spurred by a suggestion of a good friend while discussing some of the gameplay elements of the Oller, they are now capable of swimming in water. All other NPC types don't do anything special if they happen to dunk themselves in water. This might potentially change, but for now the Oller are definitely special in this regard and behave differently when they find themselves submerged.
Swim Oller, swim! They blow bubbles while in the water, like the player. Still to come is causing a splash and sound effect on enter/exit of a body of water.