Salt water and sealer
A: Thin-set mortar would be fine if you are not experiencing freeze/thaw cycles. If you experience freeze/thaw, we suggest using Flexbond Crack Prevention Mortar (Click to buy: http://amzn.to/TV2oJV), or similar, as mortar.
To protect from salt water, DuPont™ StoneTech® has an excellent Salt-Water sealer. (Click to buy: http://amzn.to/1ov1IHG).