Maritime of course has more than one meaning. If you're talking strictly Maritime Canada (which by the way excludes Great Big Sea as Newfoundland is not part of the Maritimes) then in many ways Maritime music is a subset of Celtic music. But it also contains Acadian music with continental Latinate/French influences. So you can say it's a hybrid of Celtic, English and French musics native to Eastern Canada, or for simplicity you can say it's Celtic music with French and English influences.

Maritime also carries the meaning of seafaring music. This would include things like Norwegian shanties. I assume this isn't what you mean.