I'm not sure if you made the decision already, but the Zuiderzee changed quite markedly during the period covered by the game. As you currently have it is about how it looked at the end of the period which is similar to how it was until the start of the 20th century. You might want to consider making it somewhat smaller as it was earlier in the period with Holland and Friesland almost linked. I'm not sure if accurate maps exist from this period, but in 1066 and for the first two centuries or so, the body of water was likely about half its later size and possibly a freshwater lake.
I should note that it has always be a pet peeve of mine that paradox games have not shown accurate geography for the time period. EU Rome was particularly glaring for the Netherlands, at that time, coastline erosion likely hadn't progressed far enough for there to be much if any feature to suggest the Zuiderzee would later form except perhaps a lake near the coast.
If you really want to get the area historically accurate, Zeeland and the Rhine-Meuse delta were also different at the time. However, determining what precisely they were like at the time would I imagine take a lot of time, since unlike the Zuiderzee it has not been dominated by a single erosion process but is complicated by many factors.