Bronx Shoreline Accessible Mainly in Dreams
Residents of Mott Haven and Morris Park sections of Bronx dream of having access to waterfront area that has been blocked by rail yards and other construction for years; access has become near obsession in densely populated community that has little open space; Councilman Philip Reed has set aside money to fix up one city park; there are various plans for waterfront access in Bronx, but most deal with areas farther north, like Hunts Point; map; photos (M)