Well, then, a quick primer on channels.

The most obvious use for the channels is to save a selection you anticipate wanting to use more than once. Your land mask, for instance. Create your continent shape, select it, then Select > Save Selection... and give it a name, like "Land Mask." Your selection will appear in the channels list as a black-and-white image. You can then quickly load it from the Channels list by ctrl-click or through Select > Load Selection...

Once you have selection saved, you can also perform operations on it. Suppose, for instance, that you want to do something with the surf a few pixels away from your coastline, and you want the effect to fade out from the edge. You could do that with Select > Modify > Feather, or you could get more control by copying the selection channel (just drag it to the "new" button at the bottom of the channels list) and doing, say, a Gaussian Blur on the channel, or using the smudge tool on its edges, or any number of other applications. I like to use the spatter filter to roughen the edges of my forest masks.

Then you load the selection and do your fill, or apply it as a mask, or whatever.

In addition, if you have a layer mask selected and you switch to the channels list, the mask shows up as an additional channel, and you can drag that mask to the "new channel" button as though it were a normal channel.

I'll stop there, but there are also lots of things you can do with photomanipulation by using the channels. If you can find a copy, the book Channel Chops by David Bieny is an extremely valuable resource. It was written in the days of Photoshop 4, but it's just as valuable now as it was then. It's difficult to locate now, being so old, but you can probably find it through an interlibrary loan.