A maritime polar air mass is a cold air mass that forms over which two oceans?

The naming convention for air masses can be a little bit confusing when using the term "polar".

Arctic would imply that an air mass formed in the vicinity of the Arctic Circle. Polar would imply that an air mass formed near the poles. That would mean that a Polar air mass should be colder than an Arctic air mass. This however is not the case.

Polar air masses do form over higher latitude waters, but not near so far north as to deserve the name "polar". The cold waters in the North Atlantic and North Pacific which are near and just south of the Arctic Circle, are the starting areas for maritime polar air masses.

You didn't ask but I will add one little extra. Maritime Arctic air masses do not exist, because once a body of water is frozen it might as well be land. The source area where one would expect a Maritime Arctic air mass to form is frozen most of the year if not all of the year. The air mass that then forms is a Continental Arctic air mass even though it formed over an ocean.