You can use either zinc plated or Stainless 316 mounting bolts. The stainless bolts will place more of a demand on the anode but this is not a problem. With a stainless bolt it is normally easier to undo the nut to change the anodes as the end of the season. It is extremly important to ensure no bilge water is allowed to wet the bonding connection on the inside of the boat, whether stainless or zinc plated the bonding cable may start to corrode and develop high resistance. Make sure you cover the connection with vaseline or similar to protect the connection.