Fortunately, with a nail, a lump of wood and a few spare hours, you can construct a sun compass.
(To do this Instructable properly, it is not absolutely vital to be wearing a Viking helmet, but it helps. It's a long time since mine last came out of the shed...)
Step 1: Make your Sun Compass.
Bang the nail through the piece of wood. No, don't use a rock, you're supposed to be civilised. Use a hammer.
Yes, really, you're done. All you need to do now is calibrate it.
Step 2: Calibrate your sun compass.
All you need is patience and a pencil.
Put your sun compass somewhere that is both sunny and won't be disturbed, or where you can make sure that you place the sun compass in exactly the same orientation each time. I stood mine on an up-turned planter, to stop folk stepping on the nail.
Spend the day near your sun compass, doing useful things to prepare for your raid (mending sails, sharpening swords, doing careful stretching exercises so you don't pull a muscle during an important ravish, you know the kind of thing).
Every so often (try for every 20 or 30 minutes, but at least every hour), mark the tip of the nail's shadow with your pencil.
At the end of the day, join the marks in a smooth curve, known as the gnomon curve.
Find the point where the gnomon curve passes closest to the nail. Draw a straight line from that point to the base of the nail. That will be your North-South line (as a Viking, living in the Northern hemisphere, North is pointing away from the nail. If you are making your sun compass in the Southern hemisphere, North is pointing towards the nail, and you must be the most utterly-lost Viking in history).
Step 3: Using the Sun Compass.
There you are, out at sea, no land in sight - where's North?
Hold your sun compass upright and look for the shadow of the nail. Turn (not tilt!) the compass until the tip of the shadow just touches the gnomon curve. The line you originally drew from nail to gnomon curve will now be pointing North - happy sailing!
Step 4: A Warning ...
It won't make much difference at first - you'll still get back to a recognisable stretch of coast and be able to find your way home by eye, but eventually it will be way out.
The solution is to create a larger sun compass, with a lot of gnomon curves on it - at least one per month, preferably one per week, and certainly one on each solstice and the equinoxes. Mark the gnomon curves clearly with the date they were drawn, and then, in future years, you can use the same sun compass year after year.
Copy the pattern carefully, carving into the prow of your dragonship, and you'll never get lost again, no matter what Bjorn's luck with at the table...