A THREAD FOR NEWCOMERS!

Yes, by placing Transparent Ponds (in the same "Earth Magic" tile, 7,19), Magic Clouds, Devil's Fog... and, in general, any special landscape on top. Moreover, I have encountered this more than once: specifically to hide a whirlpool and confuse the player, the authors sometimes mask the sea area so well that you wouldn't even guess there's a sea there, because you see Magic Plains or Fire Fields in front of you.