Huh. Could you enter your Oneiros while projecting yourself as a ghost?
In any case, the only thing down in the Ocean that could mess with you would be the Leviathan, and he just enforces the laws of the realm. Plus, one of the domain laws is "Leave other travelers in peace", so Leviathan would actually keep meddlers away. Hell, considering that another of the laws is "No obscuring vision", it miiiight be inclined to actually help a Mage attempting to see the Truth (no way in hell I'd count on that assistance, of course). The worst issue would be retaining your identifiers again when you re-arrived in the Ocean after crossing your Threshold and rocketing back to the surface. Assuming you came back at all, of course.