It's a fur marking that was supposed to help accentuate the belly, let the shading in that area be darker, and distract from the fact I don't draw the bits that go down there. Have since discovered it tends to have the opposite effect, and am just too stubborn to change the design.
Mostly because if it wasn't there, or was moved up to her belly button, there wouldn't be a problem. The kind of folk who think it's lewd are the kind of folk I'd be uncomfortable around in the first place, because their first reaction to seeing a naked Otter isn't "Otter!" but "NEKKID!"
Grumpyfish