Before you get out of sight of land, check the magnetic compass against known objects, using a range if possible, paying particular attention to the longest headings you'll have to steer. The deviation card may be incorrect, or missing.
You're kidding, I presume... :-) Besides the one on my own boat, I haven't seen one of those in years...
(this may mean less now with electronics and GPS than it did when I did most of my deliveries, as a teenager in New England in the '60s when my dad was a yacht broker. Usually I had a brand new boat with an un-swung compass as the sole nav aid. But it really helped back then on occasion when i found as much as a ten-degree error, which can really get you lost on a long leg.)
Yup, sounds familiar...
When I started out, running Marine Traders fresh off the ship from Taiwan, the sole navigation instrument aboard would be an Airguide "Saturn" compass, still stowed away in its box with a few life jackets, a flare kit, and a couple of fire extinguishers (Never did quite get the logic of naming a compass for a different planet
than the one you're on, but what the hey...)
First order of business upon clearing Manasquan or Barnegat Inlet, was to set a course as straight as possible down the beach, and then duct tape the compass in a matching alignment to the proper magnetic heading...
It was pretty spooky, sometimes, how far askew that mounting bracket might be :-)
I thought I'd died and gone to heaven, the first time I delivered a boat that had a depth sounder... I reckon I made the trip South a few dozen times without one...
When I finally started running boats with autopilots, then I KNEW
I had truly found Paradise :-)