Marine Digital Receivers
These units are getting more popular than regular marine radios because of its compactness and simplicity. You will still get AM/FM, Bluetooth, USB, and Sirius XM for sources. The only aspect you will lose is the ability to play CDs which is becoming less common nowadays. They are fairly simple to install and use with a digital display for all song information.
Traditional Marine Headunit
A marine headunit differs from a regular radio in that the internal electronic parts are all coated in a layer of specialized coating to prevent corrosion. They will be warrantied if the unit stops working due to corrosion related issues. Some of these units will have CD if that is a requirement for you. They do take up more room than the small digital receivers, but they are the best sounding option.
Hideaway Bluetooth Unit
This is a hideaway bluetooth unit with simple power connections and RCA outputs to go into an amplifier. It bluetooths to any smartphones and then all music and volume selection will be done through the smartphone.
Regular radio with Marine cover
This is not a recommended method for installing marine headunits. Although it does work in places where it will not get wet, but the internal mechanism can still get moisture and will not be covered under warranty for non marine radios. It is definitely worth it to get a marine grade radio for boats.