Prescott's book and the maritime boundaries map at
www.maritimeboundaries.com show a couple of bits of the high seas that
have come into existence purely as the result of boundary treaties.
They're at each end of Chile's EEZ. The situation can arise when:
a) The land boundary reaches the sea at a concave part of the coast
b) The maritime boundary defined by the treaty is *not* the equidistant
line.

At the Peru/Chile boundary, the equidistant line runs out into the
Pacific in a SW direction, but the maritime boundary is agreed by
treaty to follow a line of latitude. There is therefore a triangle of
sea lying between the treaty boundary and the equidistant line that is
within 200nm of Peru, but cannot be claimed by Peru because it lies
south of the agreed boundary. Only part of this triangle lies within
200nm of the Chilean coast. So there's a little wedge of the Pacific
that is within 200nm of land, but which cannot be part of an EEZ.
Since the boundary runs straight out from the shore, the same situation
must pertain for the territorial waters of Peru and Chile.
In the south, the treaty boundary between Chile and Argentina runs SE
into the south Atlantic after some complicated skirting of islands in
the Beagle Channel. The agreed line lies west of the equidistant line
dividing the southern Chilean islands around Cape Horn from the
Argentinian part of Tierra del Fuego. So again there's an area of sea
outside the Argentinian EEZ which is within 200nm of Chilean soil but
cannot be claimed by Chile. (This situation doesn't necessarily also
apply to the territorial sea, because of the way the boundary behaves
around the islands at the east end of the Beagle Channel.)

Interesting to see three nations each claiming their maximal EEZ but
between them managing to leave claimable areas of high seas unclaimed.

Grant