I have just re-engined my boat after nursing a very tired and worn engine for the last 10 years. During that time I had the head
off twice and found that it was quite feasable to work on the top end with the motor
in the boat.
You will find that the engine oil
will be very sooty and dirty! It will get every where.
I have rebuilt many motors over the years. You may find that a "top end de-coke" and head service
will restore considerable performance. It is feasable to do this work with the engine in the boat.
The problem you have is that if the rings need replacing there will be considerable wear in the bore and probably the main and big end bearings. Wear in the bore will involve a lip at the top of the cylinder which must be removed or the new rings will break when arriving at the top of the stroke.
Also if replacing the rings the bore should be "honed" so that the new rings will "bed in". I would recomend that if work on the rings is to be carried out the engine should be removed from the boat so that it can be fully cleaned. Also if working to the motor
to this extent it is worth replacing all the oil seals
I think you will find that it is a lot easier to remove the engine if work on the bottom end is to be undertaken. Certainly the end result will be a lot better.
Good luck with the rebuild