If I had my own room for displaying the castle I would have carved the mote, painted the bed of the mote blue, and filled it with real water.
However, for now it resides on the living room floor in full access to my wee ones, so having real water (or even gel) in there just isn't feasible at this time....too messy.
One other alternative I may use is to place a play mat under the base that is slightly bigger, and just paint blue mote water around it to suggest real water for now.