Doesn't even have to be direct bury; the NEC assumes that anything underground is wet. But, yes, any cable that can be installed in the ground can be run under a small pond.
I agreed with the above comments. It sounds like you don't need a submarine cable for your application and normal cable will do the job well. However, you may consider to specify thicker outsheath rather than the minimum standard by IEC depending on your requirement.
Most 15 KV cable is not all that tough. Are there boats in the pond? Boats with anchors? People with Ice drills in the winter time? Physical damage has to be considered. Being out sight doesn't mean a cable is free from damage.
Most medium voltage cables are installed in the ground where they encounter water. Most designs are water very resistant, some are water rejecting, and others are water tight. I would consult with your manufacturer but, I think you will find that most standard medium voltage cables will fit your application. I agree that if the cable will be exposed to possible physical damage you will want to protect it somehow.
Regards,
-Ben
Benjamin Lanz
Vice Chair of IEEE 400
Sr. Application Engineer
IMCORP- Power Cable Reliability Consultants
