Back in the day on board electrical requirements were modest but as boats have become more complex their need for reliable electrical systems have increased. When I stated cruising my outboard was started by pulling on a cord, there were no navigational instruments that required electricity to function, the boat had one dome light in the cabin and a tricolor navigational light was about all that I had. Things have moved on and electricity is needed to start the engines, power up the myriad electronics and light up the cabin interior, in fact without electricity most boats are dead in the water.
Batteries are an essential part of the boat's equipment and they need to be kept properly charged. There are several ways of charging batteries but many boaters have an on board battery charger that operates from the mains when the boat is plugged in at the dock. This is a good approach and means that the batteries should be in tip top condition and fully charged when you arrive for a day on the water. To keep the batteries charged one might be tempted to nip down to the local auto parts store and pick up a cheap battery charger but this could be a big mistake. Auto store battery chargers, the type with a plug on one end and a couple of crocodile clips to connect onto the battery posts at the other are of what is known as the ferroresonant type. They are simple affairs hence the cheap price and work by putting out a large charge which diminishes as the battery comes up to charge. These chargers work great for a very occasional charge of a partially flat battery but continued use day after day will almost certainly lead to premature failure of the battery aboard your boat because unlike a more sophisticated charger the output is virtually unregulated from a cheap auto store it is possible to boil off the electrolyte from lead acid batteries.
Marine batteries are often of one of three different types, conventional flooded lead acid, like those on your car, GEL where the electrolyte is a jelly like substance and thus cannot spill and finally AGM which stands for absorbed glass mat; the electrolyte is suspended in a glass mat between the plates allowing the batteries to be mounted in any position even on their side if desired. The point of all this is that each of these different types of batteries have slightly different charging requirements which cannot be met by the cheap auto store battery chargers. For optimum charging efficiency a three step charging process is needed; bulk absorption and float which a good marine battery charger offers. Assuming that a battery is heavily discharged the charger will go into the bulk phase where a large amount of amps are input into the battery to restore the majority of it's lost capacity. As the capacity of the battery starts to reach about 90% of fully charged the charger will go into the absorption mode, this is because putting the last 10% or so of charge back into a battery is the hardest; the charger will put out less current and the micro processors in the charger monitor the battery and vary the output until the battery reaches full charge at which pint the charger will go into float mode ensuring that the battery does not self discharge and will remain completely charged almost indefinitely until the owner next turns up at the boat and unplugs the shore power cord to head off out for a cruise. Many chargers also have temperature sensors and as such will vary the output of the charger dependent on the ambient temperature as the cooler the battery the more readily it can tolerate a higher rate of charge. Marine battery chargers are intended to be permanently installed and connected to the batteries inside the boat, they must also be wired in correctly and although you can do this yourself is best left to marine electrician unless you are confident of your abilities. The picture at the top shows a couple of marine chargers from Charles industries which are excellent products and can be programmed to charge all types of marine batteries.
Incidentally if your boat is not equipped with a volt meter on the electrical panel you can still check the state of charge with a cheap digital meter from an electronic store. With meter set to volts and the probes touching the battery terminals read the voltage. A fully charged battery will read about 12.75 volts, a battery that is showing 12.5 is approximately half discharged and contrary to what you might think a battery that reads 12 volts is in fact flat. Incidentally readings should be taken on the battery with the charger turned off and the battery allowed to 'rest' for at least 30 minutes.