Basically, servicing these old BMC engines is relatively simple. I used to do it on an old ex plates London taxi which was owned by my father. The engine is similar if not the same as the BMC boat engines although dad's was two litre. I am assuming it is a diesel. If not you probably have an SU carburettor so just take off the bell top, wipe the piston and inside the bell and replace then top up the dashpot in the middle of the carb. You could give the carb a tweak using the nut underneath the carb but you need a vacuum gauge or a good ear for an engine to do this.
Change the oil and oil filter regularly, replace the diesel filter about every two oil changes and bleed the diesel pipes, check the water pump belt/s and replace if necessary, check the secondary water pump impellor preferably take it out when the boat is laid up for the Winter if it is neoprene. If you are using blue antifreeze in the primary cooling system, change it every two years and give the block a good flushing if you can and use at least a 50/50 mixture and use rain water when refilling if you can. Check the tappets and adjust if necessary, check all flexible pipes and replace if necessary. Leave the injectors and injector pump alone unless they are playing up otherwise it will only lead to grief. Let a proper diesel engineer deal with these if necessary (costs a fortune!).
Don't forget to check the oil in the gearbox and fill up the propshaft greasers and don't forget to check all your fuel lines for damage and replace if necessary.
Hope this helps.
No doubt there will be a few more DIYers along to give you more advice but it is not rocket science. Just take your time and make a good job of it.
Best wishes.
Troutbridge.