I always just figured the offset in my head. Ships move very slowly compared to helicopters, so you can head nearly to its present position, with an offset of a couple of miles or so. If you get within 5 miles you can see it easily. I'm used to being told a rig, or ship, is in a block, which is comprised of 9 square nautical miles, and no more precise information. If I head for the center of the block, I know I can find the rig. It's hard to hide a large ship or drilling rig on the ocean.
SASless is right, they're never where they say they are. I've received coordinates that were a degree off, and it's much easier to hide a ship when you have that large an area, and especially if it's dark.