edit admin.json in permissions folder and add: "watertools": true (being careful to keep the same syntax as other entries)
save the changes (save the modified admin.json)
note: the watertools entry is not listed, but can be added.
note: in server.properties file, make sure it is set to: Permissions_AdminsFullPermissions=False (otherwise your admin group permission change will not be used if your steam UID is added to server.properties as admin)
Once done, either reboot the server OR open console and enter command reloadpermissions
red51 - as with java version, if Permissions_AdminsFullPermissions=True, full permissions are not actually used and bypasses any entries you make to your admin group.
Galochka I attached the admin.json file I edited to include watertools and other commands that are not listed in the example file for reference