There are two kinds of seals that live and breed in the Wadden Sea National Park: grey seal and common seal. They can be spotted both in water with their heads sticking above the surface and on sandy beaches where they lay and sunbathe, sometimes in large groups.
Since the seals are wild animals and they raise their pups in this area, you should be respectful while watching them. The most sustainable way to do it is during an organised boat trip from the sea. Another way is just to hike to the beach during tidal flats. But in this case, keep the distance not to disturb the animals.
The seal safari tours usually run from June through September. The best location to start from is the mainland town of Ribe towards Mandø island, with the highest concentration of the seals on the Koresand sand bank south-west of the island. Another place where you could start your boat tour is Esbjerg, with large numbers of seals sunbathing on the shores of Fanø Island.