As I was thinking of the 50/200, one of the thoughts that I had was that these were the depths that a person could potentially swim to. It is possible(though not likely) that someone could swim to 50 feet without any gear, then 200 feet is the most they could go to without some specialized gear that wouldn't normally be available in a medieval/ dark ages setting.
I've personally gone down to 25' without any help- I don't think that I can hold my breath long enough to go deeper and still resurface, and the deepest that I have dove with gear was 100'.
The general technology level for my world is equivalent to about the mid 12th century with some areas being slightly beyond, or behind.
I have to agree with RPMiller - using the diving ability of people is a perfectly rational way of determining what depths to map. In fact, it immediately gives you information for when your players are out on that water, and they decide to get wet. Brilliant.
I have to say that your map is absolutely gorgeous! The choice of font gives it a very definite character, too.
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