Water and wiring do not mix - ever! Do not route your wiring through the bilge. The oil in the water will affect the insulation and get into the harness. Always route as high as you can - under the side decks is good.
Keep your wiring as short as you can. going through bulkheads is better than going round them.
Calculate the correct wiring gauge so that you do not get voltage drops and wire heating up.
Do bind your wires together into a harness to stop them rubbing together. Remember that the wires on the outside will take all the rubbing and need protection. Wires do move far more than you would think on a boat.
Do not let the wiring hang. You will need to clip up your harness every 300 mm .
Do not force your wiring into sharp bends. This will stretch the wires on the outside of the bend and will break the strands inside the wire. So use nice open loops when you route your harness.
When calculating your wiring runs remember that you will need to route your wires once they reach the engine room. Make allowances for this and remember to clip and protect those wires also.
Remember to allow enough wire to route it around your engine. This wiring also needs clipping up and protecting.
All sensors need a good negative return to your battery system/s. Sensors that screw into your engine need you to ground the engine to the battery or their own return wire.
Do not run engine room based equipment directly from switches, use the switches to trip relays.