Was Poland a member of the Landlocked and Geographically Disadvantaged
group in the Law of the Sea Treaty (UNCLOS III) negotiations?

I doubt it, since Poland is involved in high seas fishery, as you can
see on this site.  Scroll down to "Conserving High Seas Fishery".

http://www.dni.gov/nic/special_endgame.html

When did the Baltic Sea dry up ?

://www.google.co.uk/search?hl=en&ie=ISO-8859-1&q=poland+map&btnG=Google+Search&meta=

Frde, of course, you are right about the Baltic, but somewhere in my
searching I read that "Landlocked and Geo. Disadvantaged" did not mean
absolutely landlocked (e.g., Switzerland, Austria), just limited so
much that the country is not involved in the fishery matters dealt
with by UNCLOS III.  But Poland is, apparently, from what I found.
Myo, AKA Ratty.

Danzig or Gdansk is locked in my memory