Players/Mobs riding an entity (typically a boat, a minecart, a pig or a horse) never receive any knockback when attacked.

Mobs and Players when riding in boats are unaffected by outside forces, regardless of what type of material the entity (mostly boat) is resting on. Knock back enchant, Punch Enchant, eggs, snowballs, arrows and fishing rods are unable to move Players in boats but can move both players and boats when separated in most cases. Same for all Mobs in boats or mine carts.