It sounds like you have a table top made of MDF (Medium Density Fiberboard), a particle board product.
To confirm what the substrate is, if the table can be extended open the top and examine the edge. Does it have a natural wood grain, or distinct layers of plywood, or wood particles compressed together? If not, examine the bottom of the table top for clues
If it is a particle board, while the image layer may be a thin veneer, it sounds like a printed image paper layer. The sitting water has seeped through the image layer and swelled the wood particles below.
This usually cannot be reversed. You could try to dry and compress the fibers again by placing a dry steam iron on the spot, but the risk is great that you would damage the image further and make it more noticeable.
Plywood would usually not become bubbly and bumpy, and a solid wood would not swell in that way at all.