The only one that comes to mind is mar, the word for "sea,"...Are there cases like this in other languages?

IIRC, The German word See means "sea" (as in one of the reputed Seven) when it is one gender (masc?) and "the ocean" (as in the one big one) when another (fem?). Can a better German speaker than I confirm/deny this? Jazzo, it was you wot started the thread...