You should not just go to the coast and hope to see seals, first of all, they are often not where you would expect them and secondly, they are often in reserves and off limits for visitors.
A quick search in Dutch does bring up quite a few options, search words -zeehonden kijken in het wild-.
One mentions that 'wad lopen' mudflat hiking often gives the option to see seals.
Other options include boat tours from Texel, from Ameland, from Terschelling. All of those are islands around the 'Waddenzee', in English they are called Frisian islands but not all belong to the province of Friesland (Frisia.)
One site mentions options to see seals in Zeeland, the area of islands on the Dutch west coast near Belgium.
Non of the sites I saw today did seem to have an English version, but there might be some.
While there are many options mentioned in the search, I would not say that one site stands out for me. In your case I would walk in at a tourist information office somewhere in the Netherlands and ask for them to arrange joining a tour.
Or just do your mudflat hiking and hope for a change meeting.
There does not seem to be a season, but there are two short seasons where the seals may not be disturbed, which is when they have young, and the different kinds of seals have different seasons. For one kind I found 'spring and summer' for the other kind September to December. In short, ask at the tourist information office, they will know for their region and likely for all of the country.