I'm not sure what sort of options are available to you in Elements but I'd make sure that you always have the "land" as a separate layer with the surrounding ocean being clear. That way you can always create a selection and do the painting (greens, tans, etc) on a new layer using the land selection so that you don't paint into the ocean...also use a low opacity airbrush so that you can continually build up the color instead of plopping down a big chunk of green with sharp edges.

Also, my tutorials have all been designed for Photoshop but I know that some people have created some work-arounds to adapt them into Elements so you might want to give that a try. I'm always here to answer questions (well, off and on during the evenings) to try and help you with adapting what you use with what I teach and then those who have used PSE should also be around to answer questions.