It depends on the boat.
For outboards boats, we sometimes show suggested electrical wiring for accessories on the plans or for the larger units, we include a separate electrical diagram to use as a starting point. See the plans description. The diagram shows connections to batteries, switches, panels and some basic instruments, a bilge pump etc.
Outboard engines are sold with an electrical harness and controls. To install an outboard is just a matter of plugin the harness and connect the control cables. We show chase tubes if applicable.
For inboard engines, at minimum, we show a standard engine with engine beds and sometimes exhaust and cooling.
For straight shaft inboard, we show shaft, prop, stuffing box, strut etc. with part numbers.
For all power boats and for large sailboats, we show fuel tanks and vents, fills, routing of hoses etc.