This is the accessory drive shaft. All starting and accessory drive loads are transmitted by this little shaft. The starter motor has a torque limiting slip clutch that normally prevents accessory shaft breakage. If you have a Lear starter you should be OK, but if you have an older GE starter beware that the slip clutch is probably rusty and seized solid. If you try to start with a seized slip clutch, you risk breaking this shaft.
Water is tapped off the jet pump.
The fuel deaerator is designed to remove air bubbles from the fuel. The engine would tend to loose power and flame-out in rough water before I built this device. Here is a good shot of Squirt 2 on her maiden voyage. Squirt 2 puts up a wall of water during a turn with the power on. Here is a shot of Squirt 2 at speed.