Looking at a map, it seems possible to kayak from Berlin to the Baltic with minimum portage. Does anyone know if this is possible or if any information sources exist?
With Baltic do you mean the Baltic Sea or the Baltic states?
There are several possible routes from Berlin to the Baltic Sea. If you can read German, here is some information: http://forum.kanu.de/showthread.php?t=2310
Here is a translation of the relevant parts:
There are routes from Berlin to the Batic Sea without portage: Either follow the Spree towards the east, and through the Oder-Spree-Canal into the Oder (which flows into the Baltic Sea), or north over the Havel, and over the Oder-Havel-Canal / Finow-Canal also to the Oder. Here you should paddle through the "Oderbruch" (https://en.wikipedia.org/wiki/Oderbruch), a nice, lonely, natural marsh area at the border to Poland.
The third and longest variant is the route over the Havel resp. Elde-Müritz-Waterway, Elbe and Elbe-Lübeck-Canal into the Trave.
Beginners should stop at Stettin, before reaching the Oderhaff. The Haff, and the way to the Baltic Sea should only be tried by experienced sea-kayakers. The same is valid for the Trave from Lübeck on, end the "Lübecker Bucht". Better stop in Lübeck here
I haven't done that specific trip, but I went from Bremen to Bremerhaven on the Weser river, and thought I would chime in with what I learned on that trip that might be applicable to your itinerary.
Make sure you study the tide tables in advance. The closer you get to the sea, the more you will have to battle the tides, which in Germany are quite extreme. When the tide is coming in, you might not be able to make any forward progress for a couple of hours, even when you are still miles away from the baltic sea.
Some of those rivers and canals are shared with huge container ships, which have the ability to displace quite a bit of water. If you pull over somewhere where there is no floating dock, make sure that your kayak is not anywhere near the water, or you might come back and it is gone!
Get ready for some surprised looks, especially on the main thoroughfares!