Among the number of projects underway in Watertown is the highly anticipated $400 million redevelopment of the old Arsenal Mall into what has been dubbed Arsenal Yards. It will feature 300 apartments, as well as new office and ground retail space, including eateries, a supermarket, a movie theater, and a hotel. One of the project’s developers, Boston-based Boylston Properties, also was behind a new Residence Inn by Marriott that opened in 2016 across from the Arsenal Yards site. It was Watertown’s first new hotel in more than 50 years. Boylston also developed LINX, a fully leased 185,000-square-foot lab space that sold last year for close to $158 million to New York-based Clarion Partners. The sale of LINX at more than $850 per square foot, as well as its roster of life science tenants, “cemented’’ Watertown as a player in an industry that’s been focused on Boston and Cambridge, said Aaron Jodka, director of research for the real estate firm Colliers International Group in Boston.