normally need to open the gearbox at the bottom and release the gears with a circlip so that the drive shaft can be drawn through the waterpump, then take off the lower housing and you get access to the water pump. I normally combine this with pulling the top part of the engine off and ensuring all the passageways are crystal clear.
Really easy on my Suzuki 2.2. Pull the propeller off and then take off the plate immediately behind the propeller. The impeller is in there - it lies in the same plane as the propeller.
You have a blockage either in the pipe going up the leg or in the channels in the head.
A length of flexible wire will be the best way of pushing up the leg. For the head you should be able to get into everything with a 10mm spanner and a philips screwdriver.
I used iron cleaning fluid from the ironmongers to really clean the channels out.