On my 2003 Crusader 5.7L MPI I/Bs ( ~~120-150+ hrs /yr), I replace the flat crossover-type caps and sometimes rotors about 2x a season and recheck the Cam Retard Angle withthe scanner, as the brass electrodes in the caps on these distributors with a small vent corrode quickly.
I used to check spark plugs every 2 years or so and replace them since they were already out and VERY painfully hard to access, but these Iridium plugs were always in very good shape and really didn't need replacing . I haven't checked for at least 3 yrs now but probably should do that this year and reduce the gaps a little more, already down from the spec 0.060" to 0.050" to help the dist caps last a little longer.
The dist. caps on my old carbed blue engines with the added Delco Voyager EST electronic distributor kits lasted about forever in good shape.
The 3 AGM batteries get replaced roughly every 4-6 yrs.I check them with a Midtronics battery conductance analyzer to monitor the CCA capacity.
I have a perfectly good multi-stage marine charger onboard but never ever use it, not once and haven't needed it, but good to know its there if needed. I never liked the idea of keeping the boat plugged in to shore power.
( Case in point : My dock neighbor's charging system on his 1 yr old boat completely burned up his ~~$400K Pursuit boat and totally burned up the very nice older diesel Cape Dory 28 Bass Boat next to it and did $14k worth of damage to mine directly across the dock . That was about 2AM last early summer.Brave neighbors saved my boat and several others.)