Seal Island is not very big at all. Its small and rocky elongated with its long axis oriented roughly north-south, measuring approximately 400 by 50 metres, and with a maximum elevation of about 7 metres above the high tide line. The island has a narrow western shelf, with the water depth dropping off to a depth of 20 metres within 5 metres of shore, and a broad eastern shelf, where the water does not drop off to comparable depths until about 20 metres from shore. Off the southern tip of the island is a small, craggy outcrop, the portion which protrudes above the surface at high tide is approximately 4 metres long and 2 meters tall.
Water temperature off Seal Island varies little throughout the year, hovering between 13 and 14°C at the surface and a degree or so cooler at the bottom where the depth is 20 m.