Has anyone tried using a marine AM/FM antenna?

might be OK if you're close in & don't need any gain, although most applications fare better with a gain antenna even if omnidirectional

I've been told that the Magnum Dynalab ST-4 is sourced from a marine antenna supplier.

I run both the Fanfare omnidirectional and Reflect unidirectional from ccrane.com. Both very good, but the Reflect at $25 will outperform the Fanfare ($90) if used directionally. Just ordered another Reflect from ccrane.com and they have several orphans at $19.95 (returned product).