@dansshin , a "blow out" is when the clean out plug comes out unexpectedly. There is great debate as to whether they might come out because they were improperly set or because of a minor defect in the plug (general consensus on this board is likely to be the latter). The jet path is under pressure, so if a blow out occurs, the plug shoots up, usually damaging the hatch to the wet locker. The damage and the impact also usually opens the locker, which (because of the safety switches), shuts down the engines. Even if the plug can be found, it won't lock any more. Even if you had a spare, you would need to fashion something to keep the wet locker closed to be able to go. Basically, your day is toast.
EZ-Locks (and some other solutions out there) help prevent this by providing a secondary lock on the plugs. So, even if the first lock fails or is not properly set, the secondary lock will prevent the plug from blowing. Very easy to install, generally. Very cheap insurance in my book.
Yearly maintenance... for boat and trailer... well, I will take a crack at that. Of course, you should check all this against the manual:
1) Annually at least, oil, oil filter, and plug change. Lube the intermediate joint behind the engines and the cables. Tighten all screws (hatches, bimini, etc.).
2) Every outing, check for gas smells/leaks and water in the bilge. Periodically test the bilge pump if you haven't seen it come on.
3) At least quarterly, clean bilge pump. Check battery water level.
4) Every fill up, add Marine Stabil (or equivalent).
5) Annually at least check pad wear on trailer and level of the brake fluid. Check tires for wear. Lube axels. WD-40 on just about every nut and bolt exposed (some would say this last one every outing, some would say quarterly...)
6) Every outing, check lights on trailer. Check brakes by unplugging lights from vehicle and trying to back up (brakes should lock). Examine for broken or rusting bolts and for holes in bottom of members.
I think that is it. I am certain my colleagues will correct where I have missed...