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