Lower unit / replace gear oil -- Yearly
Fuel filter – Yearly particularly on Yamaha 4 stroke 150
Spark plugs – Check annually and or when performance drops and replace plugs are worn or every 2 years ( optimax requires very expensive plugs but do not try to cheap out on those plugs use the factory recommended or the results will be catastrophic .)
Impeller/water pump – every 2-3 years or when smart guage says you are losing water pressure at Idle ( refer to manufacturers specs ) on Optimax engines check filters and screen on intakes and poppets before changing just impeller ,also check thermostats on opti if water temp will not reach 125 degrees at idle failure in this area will result in cold seize engine failure ( almost always Number 6 cy )
Primer bulb and hoses – check annually , Mercury says gas formulations change frequently and last years Alcohol resistant may not work with this years Blend
Inline fuel/water separator - I've never had one of these, but getting one installed. Any maintenance on this needed? Mercury does not recommend after market fuel separation systems , I do not use them factory has them on most your 125 2cycle does not have a factory separator on it .
Motor oil - mine is 2-stroke so no changing.... just adding but do not scrimp use manufactures brand each has unique additives to that Brand of engine components
Trailer bearings – check grease for water intrusion every use ? Not just ad but change annually with Dexter hubs pump til new lube comes out on others disassemble and repack annually
Trailer Tires - check air pressure including spare every use !!!!!!!!! biggest cause of tire failure under inflated tires , some manufactures Goodyear is one recommend 10 lbs over if used over 65 mph , second cause is failure to check tires age …circle with 4 digits indicates week and year of manufacturer after 6 years you are driving a time bomb waiting to fail
Other? May answer your last question grease engine and steering zerks when ever you do the axles You may play Russian roulette by following other procedures but I have been doing this for 60 years on I/os , Fast Boats ,Bass Boats , Multi species Boats , with zero failures other than manufacturing defects from Carlises tires .