I cast raise land on some patches of water, and got the result you see in the screenshot below - clearly not correct.  Presumably if it's cast on water, the now water boundaries should either become cliff or beach, rather than just converting to regular land.  

Along the same lines, there should be a way to make beaches - either by casting raise land on water bordering other land, or by casting lower land on a cliff, or both.  That would provide more interesting opportunities for the sea invasion option...

http://dl.dropbox.com/u/8683962/raiselandnobeach.png