Nelson C. Smith, 87, has had plenty of water pass under his keel. And observed many sharks off his bow. The retired Edgartown charter fishing captain, who has had many jobs on the waterfront, predicts an increase in shark sightings in Vineyard waters. As long as the seal population continues to rise around the Vineyard, Mr. Smith said he believes the seal’s worse predator, the great white shark, will also increase, as it seems to have done around Nantucket and certain areas of Cape Cod, according to recent reports. “More seals are showing up at Muskeget Channel.