Evil Sea Elves? Nifty idea...

What about having the Marrow and the Sea Elves being the same race? It's sort of like the Sahuagin having their normal race but with mutants who look like sea elves, except for your seas it's the Marrow that are the mutant Sea Elves. Like shock troops. Or have it so Marrow are the only amphibious, but the Sea Elves are powerful spell casters and can take some ocean with them when they attack?