getting lost at sea
Most times you just want to boxed text the getting lost part --- i.e., it just happens. A bit of the old deus ex machina, though if the characters are simply passengers on someone else's ship (who then gets lost) the logic doesn't strain as visibly.
If you are desperate for mechanics, there are third-party rules for navigation. Check out Skull & Bones (Green Ronin) and Broadsides (Living Imagination). I'm more familiar with Broadsides, which has extremely detailed navigation & piloting rules --- probably too detailed for the task at hand but its a good read.
Looking forward to the conversion!