Yes, a shelf underneath the main table surface would get wet whenever you water. You could store unused bonsai pots and depending on your angle to the sun and environment, might be able to set up a second shelf for trees that prefer a bit more shade, like dogwood, azalea, Winter Hazel and beech.
Many died-in-the-wool suiseki enthusiasts store their rougher stones outdoors too. It helps young stones develop a patina of age.
It would not be a good place to store your bonsai magazines.
