Should be perfectly safe. The shells do not contain salt but calcium. Over many years, they will degrade and slowly leach calcium which may make the soil slightly alkilin, but no cause for alarm.

Libby, I have a small area of seashell mulch. I'd suggest you put some sort of barrier down before applying the shells. They are a pain to weed between.

Even with the barrier, there will be weeds. The leaf litter and dust will eventually create soil suitable for some weed or another.