you could buy one and take the circle off the botttom and exstend the wire
Ships are pretty big. I think that loud crunching sound, followed by your submersion in cold water, might possibly reveal the location of the ship without any need for a metal detector. If you do want to build your own, try typing "metal detector plans" into Google and following any of the first three, five, eight, dozen, links. You might be surprised that there's ships information out there if you just try to find it.
You mean there's stuff on the internet that's not porn? When did they start doing that?
You don't usually try to find ships with metal-detectors, kelseymh has given you good guidance on that. What do you actually want to do here? L