There are two main types of Antenna supplied for FM/AM reception in a marine environment.
The first is a "Ground Dependant" Antenna. This is designed to be used where the antenna base is in contact with a metal surface such as steel or aluminium that has a surface area of 0.5m² or greater. This is a ground plane and the antenna depends on it for reception signal strength, if fitted on a vessel without this you will get very poor reception.
The second is a "Ground Independant" antenna. This is suitable for non metal vessels and a ground plane is not required.
Ensuring you have the correct type of antenna for your vessel is something that is missed sometimes as it is not well advertised.
VHF splitters are another option but we do not recommend them for metal vessels, as you are relying on the short wire run from the stereo to the splitter and this is generally inside the vessel so reception is limited.