Hard to tell from the photos, but one end or the other of the motor housing will come off. Probably the end you are showing.
So, remove the end of the motor housing and slide the rotor and shaft assembly out of the motor. BUT!!! When you do that the brushes
are going to fall out. You will have to avoid dropping them into the bilge
plus they are spring loaded and if you don't know the trick are a huge pain to get back in.
Also, unless you just have stuck or worn brushes
there's not a lot you can DIY repair in a motor. The common failure modes are bad brushes (which you can fix), open or shorted winding (that you cannot fix) and cracked magnets (which you cannot fix).
If the motor is dead anyway I guess you don't have much to lose by taking it apart but it may not get you anywhere. Might end up cheaper just buying
a new motor from Jabsco
. Maybe a local alternator/starter repair shop could rewind the motor if that's the problem but it could cost more than a new motor.