For the sources just look around. You have them all right under your nose all the time
Source 1: Orimazd posted a map reconstruction of coasts in AD 100 and AD 900
(Similar "all land" maps of Netherlands are all the maps reconstructing the coast for the period before AD 1000)
English article of Frisia on Wikipedia hosts a GIF image showing expansion of Frisia kingdom in the first millenium, and what a surprize, it's too showing Netherlands as "all land".
Source 2: Martmol posted a baroque map of coasts.
Source 3: Google maps + satelite views of modern coasts.
Put the maps in chronological order and you'll see the development.
Both Orimazd and Martmol brought a proof for their opinions in the form of maps, showing exactly the coasts in various times and proving enough how the coasts were changing.