If you are thinking of taking fire works during NDP or Fire Works Fest, then think it might not be as easy. Due to the close promixity of the fire work, Sheares Bridge is usually closed for traffic, photographers also will need security pass to go up (for NDP).
Also, during NDP, don't think the bridge is opened for public access due to the performance by the soldiers that go upside down rappelling down the bridge and the fire curtains during one of the performance. Thus, it might not be an easy access for the general public.
But you sure can give it a try. There is a series of stairs to walk up near Temasek Ave / Suntec area, the gate is opened for certain period of time only (normal days). But it's a long way distance of walk!