The baltic has tons of wrecks. Because of its brackishness, both marine and fresh water wood decomposing organism dont survive there and thus old ships got preserved really really well. Some are in really shallow (walkable) water, especially in areas where the land is rising.

That may be not that amazing for a shipwreck in the Øresund. According to Wikipedia, its maximum depth is 40m (130’) (https://en.wikipedia.org/wiki/Øresund), so chances are a lot of it is less deep. There also may be sampling bias, with shipwrecks in shallower water being more likely to be found, and, if the tides flow faster in deeper water, survivorship bias.