Your mother was right on some counts!! Mushrooms very easily can get waterlogged, in which case you have to cook the water off of them (dry sautee is the best method for this imo). But that's not saying they can't be washed without being waterlogged. Just don't soak them in water, water contact as short as possible and pat them dry afterwards. Only wash right before cooking too, once they are wet bacteria will start breeding fast and they will be ruined after x hours. Some mushrooms we also use a spray bottle and cloth with and that works well but seems to be more time consuming.