It's my experience that most car radios are pretty good radio receivers and since most 'marine' am/fm radios are just car radios with white escutcheon plates
they should just work.
My own preference would be to have a separate am/fm antenna rather than splitting the VHF antenna. Depending on your cruising grounds, you could even just tuck 4 ft of thin wire in your cabin headliner
as an antenna and you'd get good reception.
The best AM antenna would be something like a long wire hoisted on a halyard
... but this shouldn't be necessary in most places.