That's a pretty tall order, especially for a post on here, but I can lay out some basics.
You also might try your local library, or online, for in depth explanations. Books by Nigel Calder, Ed Sherman, and John Payne are good basic references.
Your boat has two separate systems - DC and AC. The batteries, engine starters, most electronics are DC, while your "appliances", fridge and boiler in your case are AC.
The inverters take power from the DC batteries and make AC power. As to your comment that the inverters are on all the time, I'm guessing that they are charging your batteries in a float state.
The first step is to understand how much power you are going to consume over time, which will determine the amount of battery capacity you would need. You also need to consider redundancy. Most boats with large power requirements will have separate batteries for starting and for "house loads", because you could drain your batteries down at anchor so that you could not start your engines. In addition, starting batteries (large current draw for a short period) are designed differently than house batteries (curent draw over an extended period).
You also need to think about how to recharge your batteries while at anchor or under way. Your engines have alternators which are intended to recharge starting batteries, but are not necessarily sized for recharging large battery banks, so you would need to think about larger alternators or a generator.
House batteries are designed to be drawn down to 50% of capacity before they need to be recharged. The simplified reason is that past 50% they disharge very rapidly.
The way to understand your loads is this: most appliances are rated in watts (a 60W light bulb) and consumption is in watt-hours, whereas battery capacity is measured in ampere hours, but the relationship is straightforward - volts times amps times hours.
Ex: a 100W 120V light bulb consumes 120 watt hours in one hour. In battery terms this translates to 8.33 ampere hours per hour. If you left this light on for a full day, it would consume 200 AH of a 12 volt battery bank; this would require a 400 AH battery bank, or about 4 golf cart group 27 batteries. You can see the numbers add up quickly.
You didn't mention the capacity of your boiler, but the burham website only lists one electric boiler which draws 50 amps at 240 V. This unit is a 12 KW load, and would be trying to draw 1000 amps from a 12 volt battery - not practical. If you are serious about trying to run this kind of load, you should consider a generator. You might have a smaller version boiler, however I would suggest it is not practical to heat your boat electrically mfrom a battery. For example, A small space heater of 1000 watts, if it operated at a 25% duty cycle would consume 500 ampere hours, or a 1000 ampere hour battery bank (about 10 golf cart batteries).
This might feel like drinking from a fire hose for you, whihc is why I suggested some additional reading for you.
