Replacing Water Pump in Merc 3.0 L
(and several smaller motors other than 2.5 L V6's)

Step 1: Remove ignition key and spark plug wires. IF you are going to use a helper method it will also help if you remove the spark plugs. (It it's an Opti I wouldn't).  
You will understand why the spark plug removal helps later.
Step 2: Put motor in NEUTRAL.
Step 3: Tilt motor to full up and engage the lock to hold motor up.
Step 4: Remove propeller.
Step 5: Disconnect the pitot hose (speedometer) at the coupler.
Step 6: If you have a trim tab, mark it for a reference to know its location at reassembly (grease pencil works well).  If you only have an anode plate you can skip this step.
Step 7: Remove plastic cap for access to trim tab/anode bolt.  Then remove the bolt (it's down in the hole).  Then remove the bolt in the cavity above the tab.
Step 8: Loosen the side locknuts, (2 on each side) as shown in Photo #2.

Step 9: After all 4 have been loosened sufficiently, pull the gear housing away from drive shaft housing until it's against the 4 nuts.  Then remove the loosened nuts. 
(* Gear housing will be free so don't let it fall!)

**Note: As you remove the gear housing from drive shaft housing, watch for the possibility of the water supply tube to stay in water pump housing as you remove the gear housing. IF it does, go ahead and remove gear housing then remove the water tube from water pump housing and re-install it back into drive shaft housing and into powerhead grommet.(You will have to push it back up into rubber grommet).

It should stay at this point, (you will need a flashlight to see up to grommet.) 
Step 10: Slide it out of drive shaft housing and place it in/on a suitable holding device or bench vice to hold it with drive shaft in a vertical position, (drive shaft pointing straight up.
Step 11: Time for disassembly of the water pump.  Remove the rubber water seal (slide it up the driveshaft) and the water tube coupling assembly (it may have stayed on the supply tube in drive shaft housing) if it's not on water pump housing.  Then remove the 4 bolts in water pump housing. (Metric bolts).

Step 12: Lift water pump housing up and off.  (May have to use 2 screwdrivers to lightly pry up if it's stuck). Watch as you lift the housing...the impeller may stay in housing (it's ok if it does), but make sure the key stays put (don't lose it.)
Step 13: If the impeller stayed put, go ahead and remove it.  (If it's stuck use your screwdrivers to lightly pry it up).  Be careful not to damage the face plate.
Once the impeller and key are off, remove the face plate and gaskets (2), one
on each side of plate.
Step 14: Time to inspect the water pump housing & face plate, also the water tube coupling assembly.  Check them for wear or any other type of damage,
(grooves or heat damage).  The coupling assembly has two O-rings inside of it.
Check them for cuts, nicks, etc., and replace if needed.

Step 15: Clean gasket surface areas of gear housing, both sides of face plate and water pump housing.
Step 16: Time to start reassembly.....Place NEW face plate base gasket on gear housing. The face plate, then NEW top gasket. *Make sure the 4 bolt holes all align; 
(if they don't, you have something upside down) as shown in photo #5.
Step 17: Install the key (use a small amount of grease to hold key in place).
It goes on the flat area of drive shaft.  Then slide the NEW impeller down and
over the key. (Make sure it all aligns.)
Next step is much easier if you have the alignment pins to install as shown in
Photo #1. (You can make them; they are (in this case anyway), 6mm bolts without heads).

Step 18: Put a VERY light coat of grease in the water pump housing insert area.
Then slide housing down shaft to impeller and align with pins if you have them.
Step 19: With one hand, turn driveshaft CLOCKWISE (as you look down from
top) and with other hand push down housing onto impeller.  Keep turning drive shaft 
until the housing is all the way down.  ** Without the pins you will have a
big problem keeping things aligned and not screwing up the top gasket.
Shows this in Photo #6.
Step 20: Install the 4 bolts finger tight and then evenly tighten them with final
torque of 60 in. lbs.  Then install the water pump rubber seal.  (They have a special tool for this too, it comes with seal kits and water pump repair kits.)
VERY IMPORTANT to get this seal installed properly to prevent air from being drawn into pump resulting in overheating.  If no tool is available, push it down until it has a height of 0.350 in. +/- 0.30 in. (that's about 11/32")
Step 21: Install the water tube coupling after lightly greasing 0-rings.  Then grease the splines on drive shaft and shift shaft (* keep grease off top end - only do sides).
Step 22: Time to install gear housing back onto motor.  Have two methods for
this..... #1 Without helper  #2 With helper....we'll cover method #1 first......

Method #1
Step 23: Put control box/motor in forward gear....then turn propshaft COUNTER- CLOCKWISE as you rotate the shift shaft CLOCKWISE (shift shaft will only turn less than 1/4 turn), until you have it in forward gear.
You will feel it go into gear and propshaft will become very hard to turn, so stop....it's in gear.
Step 24: Time to install gear housing, so slide your prop back on temporarily.
Step 25: Slide gear housing back up into drive shaft housing as when you removed it.  
As you get it up near the top you will need to feed the pitot hose through the hole in driveshaft housing and as you get it closer you will need to rotate the prop COUNTER-CLOCKWISE to align splines on drive shaft to the crankshaft.  After it aligns, you stop turning prop and make sure the shift shaft aligns and slides together.  Once all is aligned push drive shaft up tight.

Method #2
Step 23: Time to install gear housing, so take it and slide it back up into drive shaft housing as you did to remove it.
Step 24: As you are near to getting it back up into place you need to feed pitot hose through hole in drive shaft housing.  Then as it gets even close you have to have your helper turn over the powerhead by rotating the flywheel a small amount to align splines.  Next make sure the shift shafts align together.
When all is aligned, slide it up tight.

Next step for both methods: 
Start the 4 side locknuts (you can stop holding gear housing in place after you get one 
nut on a few turns).
Step 25/26:  Snug up at least one nut on each side to hold it up tight in place.
Then check shift operation.  With Method #1, it's already in gear, so shift to
NEUTRAL.  Prop should spin free in both directions. IF NOT, you must redo
Step 24.  With Method #2 - Temporarily install prop and have helper turn it
COUNTER-CLOCKWISE as you shift into Forward gear, it should go into gear and stop turning.  IF NOT, redo step 24.  
** Never shift into forward or reverse without having shafts moving!!
Can cause damage!!
Step 27: Everything works right.....so finish tightening 4 side locknuts.
(Torque spec is 55 ft. lbs.)  Then install bolt back under cavitation plate in trim tab recessed cavity - torque to 45 ft. lbs.  Then install trim tab or anode plate and torque 
bolt to 40 ft. lbs.  Put plastic cap back in access hole.

Step 28: Pull prop off if it was on, grease prop shaft and reinstall prop and all hardware, torque to 55 ft. lb.
Step 29: Reinstall plug wires (plugs first if you used Method #2).

Final Note: If you used Method #2, the reason to remove the plugs is so you can turn the motor over easier without building compression.

Well......it's time to find out how your job performs.....TEST RUN TIME!!