Kayakers, canoers, boaters of all descriptions and waterfront activists swelled the Island crowd to a record 10,674 on the second annual City of Water Day, July 18, sponsored by Metropolitan Waterfront Alliance. Including ordinary landlubber visitors with no idea what they were in for, and students at opening day of the New York Harbor School's summer session, it was the Island's biggest single day since it reopened three summers ago. One more thing: Yankee Pier reopened. Bigger boats can now tie up there - research vessels and historic ships on exhibition.