Best way is to open the map in a different tab or download the picture so you can zoom in to see the dots. Take the map with a grain of salt, it doesn't represent the 'exact' location but a general place to look for shells.
Seashells can also be found along the coast of Solstheim (no map as of yet). Solstheim is a good hunting place for seashells. The distance between the spawning spots is generally shorter than on the northern Skyrim coast and they are often easier to see because a large part of the island shores have dark lava sand and less clutter in the way.
Shells are mostly near wood trunks or fallen trees, sometimes near lonely rocks too so keep an eye out for these in the wilderness, shells don't always spawn at the points on the map, there's a ~75% chance one of the dots has a shell on it.
Here some example pictures of shells found near dead trees:
If you don't feel like finding shells legitimately you can write coc aaaSeashells and you will find yourself in a room with every seashell available: