Neil Sims tends his rowdy stock like any devoted farmer. But rather than saddling a horse like the Australian sheep drovers he grew up with, Sims dons a snorkel and mask to wrangle his herd: 480,000 silver fish corralled half a mile off the Kona coast of Hawaii’s Big Island. Tucked discretely below the waves, Sims’s farm is one of 20 operations worldwide that are trying to take advantage of the earth’s last great agricultural frontier: the ocean. Their offshore locations offer a distinct advantage over the thousands of conventional fish farms—flotillas of pens that hug the coastline.