the sink is stainless steal that would just need a little water sealant around the edges. 

Perhaps some stain/sealant is better than nothing. 

Like mentioned above, make sure to use killz, or some other sealant to cover the subfloor.

The mason had mentioned that because of the freeze / thaw cycles in NYS and the sealants ability to seal and trap moisture, that he has done significant repair work.  

There is often damage below roof valleys.Damage at trim (usually from failed sealant).Damage from miss directed from sprinklers (years of direct spray can affect many exterior wall coverings).  

Instead of the more typical flat concrete roof with waterproofing sealant, this one is built with concrete slab laid on a slight incline, with what look like asphalt/composite shingles over top, and the shingles painted over with a waterproof coating!

Install Tyvek or roofing felt, then hardie backer, then tape and mud the seams and corners, mud the screw holes, and roll on a waterproofing sealant, like RedGard.

Don't forget a case of roofing nails and some type of roof sealant for the oops moments and possibly some metal flashing for chimneys and any valleys.

and one of the HD associates recommended Fusion Pro Single Component Grout as the grout doesn't require any mixing, is a consistent color, and doesn't require sealant.

The waterproofing he is talking about is a clear sealant.