An employee was working on a suspended walkway with handrails, located under the drill floor. While in the process of installing the bolts in the flange between the bell nipple and the riser spool above the BOP he fell outward of the walkway striking the deck below, and then falling into the water. The employee was installing the bolt furthest away from the walkway and was attempting to install the nut on the bolt. It is believed he positioned himself between the top and middle handrails in order to reach the bolt and start the nut. At this point his entire upper body would have been outside the handrails.