NOTE: these are quick, cheap
and dirty suggestions to get you some sounds. There are "better" more elegant, technically better solutions that will greatly improve your signal and range but these will work.
If you have a marine
on the boat you can share that antennae with the FM stereo by using this
2 Way Coaxial Switch
VHF / Stereo Switch
Used them on my boats for years and had no problem and no noticeable loss of range on the VHF
If you don't have a marine VHF
on the boat then select whatever type plug
fits the antennae connection on the back of your stereo and run the wire out to a convenient location. If your connection and the wire you use is shielded coax (the stuff with a thin wire in the middle, then a layer of insulation
all wrapped in a woven wire covering) then you need to strip some of the woven covering off the outside since the single
goes into the radio
on the small, center wire.
Height is range for FM so if you want to pull in more stations raise the end of the wire. You could wrap the bare end of the wire around a SS shroud
or stay to get a higher antennae.