Yes it is true that cats and dogs can swim but the question is for how long? If for some reason your pet should fall overboard while you have your attention drawn to something else it could take some time before you notice and are able to search for your friend. Your pet will be very low in the water and thus may not be able to see land. So they may swim in circles until they tire out and drown.
Keep your pet safe if you take them with you in a boat.
