As a working bricklayer I can give you a few hints. I would use type S mason cement and a fine grade sand. Use a mortar water proofing additive when you mix your batch, (Dry Block, Laticrete). Use a hard fired brick and make sure all mortar joints are completely full ( bed and head joints) keep all mortar joints 3/8" - 1/2" in width. Use a round jointer to seal and finish joint. Seal all masonry with waterproofing agent after mortar has cured. You may have to do this every year.
Hope this helps.