Mercury Mariner outboard motors are well known to professional and recreational boaters alike and have a well-deserved reputation for reliability. One of the few parts that will wear out even in a well-maintained outboard is the water pump. Fortunately, it is easy to see when the pump is no longer functioning and almost as easy to fix. When water is no longer being ejected from the cooling water exit port, first check that this opening, or the intake at the bottom of the motor, is not clogged. If these are open, a new pump impeller is probably needed.
Repair the water pump with the outboard on or off the boat. If on, the boat must be out of the water and suitably supported on a trailer or stands. Make sure there are three to four feet of free space beneath the lowest part of the outboard to allow the drive shaft to clear during disassembly. If off the boat, the motor can be mounted vertically on a suitable platform or can be placed on its back on a flat surface.
Loosen the four bolts that hold the gear housing assembly (containing the propeller) to the drive shaft housing. You will find the heads to these bolts pointing downward, between the propeller and the horizontal blade or fin. Remove the bolts completely; then gently pull or push down on the gear housing until it separates from the rest of the outboard. The housing will have the drive shaft attached, so continue bringing the assembly downward until the shaft is clear.
Place the gear housing in the upright position -- as when the outboard is running normally -- before the impeller is extracted. If you don't do this the gear case oil will leak out. The impeller housing is usually plastic and is held together by four bolts. Loosen these with an appropriate socket wrench and remove them. Gently pull the top of the housing upward and gently pull off the intake tube. Now flip the housing over and you should see rubber impeller. If the impeller has lost any of its vanes, make sure they and any other parts of the impeller are removed from the impeller housing.
Scrape off the old gasket material from the bottom of the water pump housing you removed and from the surface it was mounted on, with a putty knife. The water pump replacement kit may include up to three gaskets. Place these as directed in the included instructions. Gently insert the new impeller into the water pump housing. There should be a stainless steel liner that it fits into. Reassemble the water pump housing using the four bolts, and tighten as per the manufacturer's specifications. Reattach the water intake hose by pushing it onto the seal.
Line up (from front to back) the cam rod, the drive shaft and the water tube as follows: the cam rod with the well immediately in front of the water pump; the drive shaft with the crankcase; and the water tube with the top of the pump assembly. Push the gear housing onto the drive shaft housing, reattach the four bolts, and tighten according to the manufacturer's specifications.
Before reinserting the bolts, cover the threads with a coat of heavy, marine-grade grease.