It's stormy, the boat is bouncing, and you just need to know which handle to turn to empty the toilet. There is nothing more helpful to you, your crew, and guests at that point than a handy dandy label. I recommend that every boat carry a simple low tech piece of gear called a label maker.
It seems obvious to my husband, “Tighten the main sheet!” he belts out as the choppy seas and off side winds bob us around like a ping pong in a bathtub full of toddlers. And to him, it's obvious. The main sheet is the white line with the little red specks. To anyone less familiar with the boat, however, the main sheet is any one of four lines which inexplicably all disappear into the mast only to emerge somewhere north of the bimini and completely out of sight. The blank look on the face of our guest as he stares in bewilderment at the lines is a very sad testament to our failure to state the obvious.
The best boats, in my opinion, come with instructions. You get on board, and you immediately know which locker holds the propane tanks and which one holds the dock lines and fenders. You glance over at the row of blocks on the starboard side and make note of the main halyard, reefing lines, and jib sheet. You do not spend five minutes trying to find the place to put your toilet paper after you've done your business. And never mind guests, there are things you only touch on your boat during a haulout, overhaul, or maintenance emergency. I don't want to fumble round trying to figure out which of three thru hulls is the holding tank outlet, shower outlet, or raw water inlet when there is a leak. I absolutely don't want to ever wonder which direction to set the holding tank Y value. Then there is the electrical panel with its airplane like array of buttons, knobs and unnamed LEDs.
Brother PT-1280. This little device runs on two AA batteries and produces a steady stream of verbal idiot lights. You should be able to find one for about 20 USD, so this is not a major investment. Do not, however, let your children play with this thing as the tape is rather pricey.
Once you get the labeler on your boat, you will be surprised how many rolls of tape you go through. The captain and I routinely leave pithy notes to our future selves to remind us not to make the same mistake twice:
Behind the head.....................If you haven't eat it, don't put it in this bowl.
Over the second water spigot in the galley......Do not make coffee with this. It's salt water.
Next to the ignition switch............Check the dinghy painter.
Under the previous one.................Count the children. You should have at least three.