Martin Pratt wrote:
"The normal baseline for measuring the breadth of the territorial sea
and other maritime zones is the low water line along the coast as
marked on large-scale charts offically recognised by the coastal
State. So technically the baseline does not change as the coastline
shifts, but it does change when those shifts are recorded on
officially recognised charts. Such changes will probably have an
impact on the location of the outer limits of the territorial sea and
possibly the EEZ as well, but agreed maritime boundaries remain fixed
until the states involved agree to change them regardless of what
happens to the coastlines between which they are drawn."

Thank you for this information. It strikes me as odd that at sea, a
once established boundary remains the same, regardless of the
changing of the circumstances on which that boundary was based,
whereas in a river, when the boundary has been established as
following the thalweg, the boundary normally does seem to shift with
the thalweg. Maybe this is because the boundary in a river is
directly based on the thalweg, and at sea the boundary is based on a
baseline, which in its turn is based on the low water line, so there
is one extra step to take.

This is just me thinking aloud; I think it's odd but not ridiculous;
if someone has some more information on the theoretical basis of
establishing boundaries in wet places, I would like to hear about it,
though...

Peter S.