Stripping the gearbox is tricky, unless you know how - here you go!

{having removed the engine, water pump pipe and drained off oil)

remove propshaft flange with 24mm socket, locking the flange from rotating either in a vice, or by using a long bar against studs.you may find the flange taps off, or have to use a puller.

now you can ....remove the water pump

remove the two 13mm bolts, with a little tapping/levering, remove the casting, pull off the gears (use a pair of screwdrivers if the woodruff key is tight on the driven gear), remove the back plate, o-ring.

to remove the input shaft, output shaft, reverse shafts, here's a synopsis;

input shaft

this will pull out easily, with a little resistance to the water pump seals and gear if still attached.

reverse shaft

this is just a single toothed wheel that taps out from behind.

output shaft (the big one)

to remove this, you have to first remove theinput shaft, and then the gear lever has to be pulled rearwards to allow removal; this may be possible without removing the lever assembly, providing the oil filler bolt is removed?

to remove lever

drift out cotter pin/split pin from gear lever. this is difficult/impossible sometimes in the boat. use a drift/heat and puller, remove the lever.

de-burr (in case the end is rusted/beaten up) the outer end and slide out the gear cam that the lever rotated(you will find there is a notch in the cam to allow it past the revers shaft's bit of the internal casting.

prize out the stiction c-ring that is supposed to slide in the cast groove, so you can clean/grease it up before re-assembly. you might have to get a rusted one moving by drifting it round with a blunt drift/WD40/heat first, then prising it out with screwdrivers.

remove the detent spring from the lever to de-rust/gunge it.