A boat marina on the Hudson River is in trouble with city authorities. The city comptroller, William Thompson Jr., said yesterday that fiscal irregularities at the 79th Street Boat Basin have “left the door open to potential fraud.” An audit by Mr. Thompson found that the parks department has “dropped the ball and seriously failed to properly monitor” operations at the facility along the Hudson on the Upper West Side. He said that since marina rates have remained about the same and the facility did not substantially expand, the revenue increase underscores that “fraud may have occurred.”