I'll second Nos on the "B" casing comments - the bit that's welded holds the bearing housing in place! When you say the "A" case is siezed, do you mean the prop / shaft don't turn, or you just can't break into them?
The funny shaped thing round the long shaft is your water pump housing. The metal funny shaped thing in front of it is what holds the bottom of your gearshift rod & mechanism in place.
Here's a pretty pic of what's inside:
Choose "Mariner" - then you get a list of every mariner ever made, scroll down & choose "55", pick any random chassis number & look at "gearcase". The "straght in" address link is too long to paste here! The prices are in Dollars, but they are also conveniently roughly what your local Yam dealer will charge you in £.
If it's the pin I think may be missing, you'll probably need to pull the prop shaft out to get it back in.
Now, if the thing is siezed shut with salt, what to do is find a big bucket of warm water (cold will also work, but warm will dissolve the salt a lot faster), and steep it for a bit. Tap the bolt heads sharply with a small hammer as well before you try to undo them, that may help to release the salt bond in the threads. worth repeating the above steps a few times before trying to undo anything, otherwise you'll need to go buy a set of stud extractors! If you take the plastic wings off you might get it in a standard plastic kitchen bin.
I'm going by memory here, but is there not (behind the prop) a couple of screws to hold the bearing housing in place? (I may be thinking of my old Suz here) Undoing them should release the shaft assembly as shown on the link digram. If not, any chance of a close-up pic (i'm nowhere near mine 'till the weekend)
If you need a hammer to get anything out just replace the bearings as a matter of course - ball & roller bearings don't like impacts! Also replace ALL the seals - once unseated you'll be bl**dly lucky if they re-seal properly.