Four short lines paint a picture of someone sinking through black water, cut off from everything. The speaker can't see or hear anything, so the world has shrunk to a single sensation: the slow movement downward. The opening image of a cloud falling through the oceans is odd and a little dreamlike. A cloud belongs in the sky, so seeing it here suggests something that has lost its natural place and is drifting without control. The speaker may feel the same way.
The mood is heavy, quiet and numb. The most telling line is the third, where the speaker is only waiting to see "if my heart will beat slower." That reads as a mix of exhaustion and dread, as if slowing down might mean giving up. The final line, "Sinking to no where but down," offers no destination and no hope of rescue. Judging by the imagery, the poem seems to describe depression or despair, though it never says so directly, and that restraint makes the feeling more haunting.