Shoal shell and shoal salt are obtained in shoal cave north of mossdeep city sometimes in the cave water floods it this is the time where you can get the shoal shells when there is no water this is the time to get shoal salt bring them to the old man to get a shell bell.

Shoal

In the shoal cave where there you find that item 'nevermeltice'

Hard to find but it's possible ;)

Note: you only can get there when is NOT under water (when you can find shoal salt and not shoal bell)

shoal

The noun 'shoal' is a singular, common, concrete noun; a word for a large group of fish; a word for an area of shallow water, especially as a hazard to navigation; a word for a thing.

The noun 'shoal' also functions as a collective noun for a 'shoal of fish'.