There are at least four ways to keep your batteries charged:

Solar Panel
This is my preferred alternative. Solar panels are quiet, reliable, and inexpensive. Unfortunately, there is limited space on a sailboat for solar panels - which must be exposed to full sun without shading from boom, etc. Still, if power is used sparingly, a very practical 40 to 60 watts of solar power will be sufficient to keep batteries topped up indefinitely. This has been my experience for the past six years on Teal.
48 volt solar panels in sizes suitable for a sailboat are not common, but suitable 50 watt panels are currently available on Ebay. Try
this link
Alternatively, two 24v. or four 12v panels can be used.
In addition to the solar panel, you will need a charge controller. I have used the
Prostar PS-15M-48v which is widely available online. It is not necessary to have a charge controller with output adequate for your EA motor. You can wire the motor directly to the batteries and use the output from the charge controller only to power your 48-to-12v converter for ship’s power.

Wind Generator
48 volt wind generators suitable for sailboats are widely available. This is another 100% solar solution, one with the potential to charge your batteries more quickly than solar panels, given the right conditions. Like solar panels, wind generators require a charge controller.

Engine Powered Generator
Not for solar purists, but potentially practical in some situations, small and quiet gasoline generators are available from Honda and others. If I wanted to take a long trip on the inland waterway, for example, I might take one of these along. It would require a converter to get the necessary 48 volts dc from the generator’s 110 volt ac output.

Shore Power
If you keep your boat at a slip with plug-in power and you use your boat primarily for day sailing, this would be a very practical solution. Because you will have all night to charge the batteries, a large capacity 48 volt charger is not necessary - a 10 amp, or even 5 amp, 48 volt charger will do fine, and these are quite inexpensive.