With AM and FM stations you would have numerous data to sample every second, not just one or two, throughout the waters of the US and, unlike with a car or RV, boats travel slow enough when cruising that things would average out. It all comes down to the triangulation algorithms AND them operating over time. The system would not forget every moment before the current data was taken.
As for large ships, required to have radar, they will still have their radar though even if you are dark (small yachts are often “dark” anyway). And your system would indicate their bearing to you and to some degree heading as your system tracks them over time ... kinda like doppler systems.
More to the point, with such a system you can be aware of big boats even when visibility is bad. Something most small yachts do not often have.
Lower draw on house power than active radar too, especially if the app can be running on a tablet or smart phone.
Has no one here ever head of Loran?
Years ago I had Loran on an airplane. Worked great, but I understand that it has been shut down.