As a ham radio operator that designs antennas, let me chime in a bit.
The THICKER the wire, the wider the bandwidth, mean the more frequencies you can do, however it appears most of you use a single wire antenna tied to a tuner, so we can throw that pretty much out the door.
Main thing with a single wire antenna is make sure it is longer than 1/4 wavelength around the frequencies you want to use (lower the freq, the LONGER the antenna has to be).
Make sure it is THICK enough to withstand getting whacked hard, taking any damage it might take from tripping over or hitting with something (hopefully NOT a boom?)
The coating on the wire is pretty irrelevant to the wire - although some types of materials have a different "dielectric constant" which might affect things at UHF frequencies (no one really uses that on boats it seems) so, pretty much no big deal.
In answer to your original question, Copper is PERFECT wire to use, in fact, other than something like silver it has the BEST conductive parameters of most things (ok, Gold and a couple other very expensive metals are better than copper, but who wants a gazillion dollar antenna when copper works great?)
Obviously, the point of having a covered wire is to prevent corrosion
will degrade the antenna's ability to radiate a signal over time and therefore, you want to keep it as clean as possible. Any connections to the antenna from coax or other feed lines should also be protected. Silicone grease is good for that.
Several people have mentioned stays as being used as antennas... I might look over some different types of sailboats and the various rigging
and figure out some good antennas that will work well... give me some time to work out this sailing stuff