@tmartin The main restriction is the Danish Strait. The Germans own it, and thus the Russians cannot move through it. So you cannot make the move SZ115 to SZ112. You can move anywhere in the Baltic sea including spaces occupied by German ships, if not at war.
There is an exception: when not at war, the Russians can ask the German player to allow the units to pass through the straits. There very little incentive for the German player to allow this, but it is permitted in the rules.
pg 9 in rule book about Straits:
If your side (but not necessarily your power) controlled a canal or narrow strait at the start of your turn, you may move
sea units through it (you can’t use it in the same turn that you capture it). If a canal or narrow strait is controlled by a
power not on your side, but with which you are not yet at war, you must ask permission to use it, which may be denied.
You can’t move sea units through a canal or narrow strait that is controlled by a neutral territory or by a power with
which you are at war.