I don't remember seeing a rule for or against this, but is it legal to muster ships in multiple seas, or in a port and than a sea?

Yes, you certainly may - provided the sea area(s) and port(s) are connected to an land area with a castle or stronghold.

Winterfell may muster ships in Port, in the Bay of Ice, and/or in the Shivering Sea. One in Bay of Ice and one in Shivering Sea, for instance.