In just three short lines, the speaker claims a kinship with the seal, an animal at home in water and awkward, or at least different, on land. The title seems to be a playful blend of "pinniped," the group of animals that includes seals, and "pedestrian," someone on foot. So the speaker is a seal-like walker, a creature of two elements.
The last line is the puzzling and interesting one. The speaker walks "beyond the version of the sea I am." That suggests the speaker is partly made of the sea, perhaps in a bodily sense, since we are largely water, or in a more personal sense, as if each of us is one version of something vast. Walking beyond it could mean stepping outside the self one usually knows.
The mood is quiet, curious, and a little wondering. The speaker doesn't sound troubled. There's a calm sense of belonging to two worlds, and of moving between them with gentle humor and awe.