If I tag channels like above, do they show up in feeds for everyone, or just for peers?
They will show up in the feeds of your friends and your friends of friends. Tags are a way to organize your posts, rather than a way to restrict or expand viewership of them. However, there may be a friend-of-friend you haven't met yet who is also into boating, and becomes a direct friend of you through exploring the #boats tag and seeing your post.
channels are not global, to get them you still need to have within your network (2 follow hops from you)