In the jam of boats on Great Salt Pond, year-round residents see reasons for concern where businessmen see an opportunity. The largest of the pond's three marinas, Champlin's, whose trident-shaped piers are visible at left, is proposing to expand into 4 acres of public trust waters. Block Islanders have fiercely resisted the plan; hundreds of them showed up at a hearing last June holding protest placards and chanting slogans.