Water culture will rot off the existing roots first. New root that form later will be able to adapt to the water, but you should do a little research on this website for a few "water culture" posts to see how others did it.
And again, the pot you showed above is "glazed" pottery and will only hold in water. Glazed pottery does not breath like terra cotta pots available at local garden stores everywhere. Rotted roots is from the pot, lack of air exchange. Many here use styrofoam peanuts (the packing kind) in the bottom of pots to create a large "air bubble" for the plant. While dendrobiums can be potted in spagnum, a mix of bark, perlite and charcoal would work much better. Orchids live on the bark of trees, without drawing any nutrients from it's host. They survive off of dew, rain and whatever falls onto their roots. Putting the same into a pot that doesn't allow for air-movement will rot the roots.
Hang in there! We all kill our share before we get a few to thrive.