Most of western Netherlands is below sea-level; but this is not land claimed from the sea.
This was bogland, organic delta soil just above sea-level It was partially habitable without serious hydro-engineering.
But the engineering that did occur as necessary and as part of claiming wetlands drainage that made land useful started the long, slow decrease in elevation to the levels that we see today.
The primary effect was simple: water is a large component of peat, so drainage removed volume.
The next effect takes longer. When peat is exposed to air, it reacts with oxygen and decomposes, decreasing volume further. This effect is continuous with any agriculture or livestock activity.