When seated in a boat, any type of projectile launched by the player cannot be aimed up or down. All projectiles launched will only fly straight forwards in front of the player regardless of how far up or down they are aimed. This happens with bows, crossbows, snowballs, splash potions, tridents, and even fishing rods. It does not matter whether the boat is in water or on land, it will still happen every single time. When entering 3rd person the player character can be seen aiming their weapon up or down but the projectile will still shoot straight ahead. The same behavior does not occur when seated in a minecart, where I can still aim freely. I do not know whether or not this is an intended feature, but it does not seem like one.  This issue occurs even when in a freshly created world with default client settings.