The simplest type of maintenance log is one that lists activities in chronological order as you have performed them. But finding the last time you replace,say, a water pump impeller on your genny may not be so easy.
A more convenient style maintenance log is by major type of equipment, and under each heading a chronological activity list. Equipment might be broken down by port engine, starboard engine, genny, autopilot, watermaker, water system (heater,pumps, fawcets,etc), electronics, electrical system (bonding, fuses, electric panel,windlass), air conditioning, autopilot and hydraulic steeringetc. This makes it easier to find when, for example, hoses on your port engine were changed. Batteries, for example, in the above structure could be with either port or starboard engine or under electrical. But you have to remember where an item is catelogued. And it takes more room in your book as you need to leave space for each item...unless you use a looseleaf notebook.
I use a spiral notebook which is inexpensive and easy to flip thru.