Does this still sound complicated?
Okay, first you need batteries. If you need to start an engine then you need marine batteries. These are very common; you can get them at Walmart and most auto-parts stores. Marine batteries have screws on top with wing-nuts to attach the wires. This particular battery is sealed to prevent spilling any acid (not a bad idea).
If you don't need to start an engine then you are better off with deep cycle batteries. These are not as easy to find. They are used in things like golf carts, electric scooters, and indoor fork lifts. These may not have wing-nut terminals. Again this particular model is sealed to prevent spilling.
Batteries designed for marine use like these: http://www.emarineinc.com/categories/Batteries/
are normally sealed while others might not be. This might be something to keep in mind.
So, how many do you need? Well, if you have to start an engine then it is often a good idea to have one starting battery and two house batteries. It makes it more complicated but you typically keep these on a separate circuit so that if you run down the house batteries you can still start the engine. Also, if you get a little fancier you can switch in the house batteries and still start the engine if the starting battery goes bad. If you don't need to start an engine then you might be able to get by with just one house battery. Many boats and RV's include just one but have room for two.
If you want to use regular 110 volt AC appliances you need a charger/inverter unit like this one:
Inverter Charger | Freedom HF Inverter/Charger | Xantrex
It can handle 20 amps which is enough for a small boat. It's like having one outlet. It will take care of charging the battery from shore power. You could also power it intermittently from a generator however you would probably need an adapter plug since a shore power plug is not like a regular outlet socket.
Let's suppose you wanted to use solar or wind power.
This diagram is not exactly right. The diagram makes it look like you only need a charge controller for solar but not wind. In reality you need a controller for either one. However, the controller and meter are often built into the same panel which is why they only show one panel for wind power.
For example, this 12 volt wind unit:
Air Breeze Marine 12V Wind Generator - eMarine Systems
includes the e10 panel:
This e10 panel is both the charge controller and monitoring panel in one unit.
As the above diagram shows you would hook the e10 panel to the battery. Since you would already have the charger/inverter you just hook both of them to the battery in parallel. In other words they hook to the same battery terminals. They will work together or separately to charge the battery.
You can use a solar panel like:
80 Watt/12 Volt SOLARLAND FOLDING SOLAR PANELS with Controller
This particular solar panel comes with a controller and monitoring panel but not all of them include this.
If it didn't come with one then you would need a controller like:
ProStar PS15 15A/12v Solar Controller - e Marine Systems
This unit is both charge controller and monitor. You hook it up in parallel with any other charger. If you had a charger/inverter, solar, and wind then you would have three chargers all hooked to the same batteries. Notice that there would still only be one inverter so even this arrangement wouldn't be too bad.
Now, this would get much more complicated if you tried to use one of their 24 or 48 volt wind turbine systems. This would require 2 or 4 batteries hooked up in series (rather than parallel) and then your charger/inverter would no longer work. As long as everything is 12 volts you can get a modern system put together without too much trouble.