Can a mask hide only one object and not the others? I want to make the word "SECRET" disappear behind the horizon. That is, I need a mask for the lower half of the image, but if I do this, it also hides the sea..! How can I accomplish this?
OKay, I figured it out:


Using the UV/Image Editor, create a mask, and give it a meaningful name;
Select the "SECRET" text strip ; Its "Blend" should be "Alpha Over" that's the default for text strips anyway, leave it at that;
Add a "Mask" modifier and select your newly created mask by its name.