500 Miles East of Canberra: Ocean, Not Sea

Canberra sits 150 km inland. Due east: 500 miles (800 km) puts you in the Tasman Sea—the South Pacific's marginal sea between Australia and New Zealand. No named "sea" sits exactly there; it's open Tasman. The nearest land: Lord Howe Island (600 km NE), New Zealand's South Island (1,600 km SE).

If you meant "what body of water," it's the Tasman Sea. If you expected a named sea like "Coral Sea" (north) or "Bass Strait" (south), the compass bearing matters.