If you can work on your house wiring, you can do this installation yourself. It isn't any more complex.
Given that you have a water heater ( is it 12VDC ??? ), refrigerator ( is it 12 VDC ??? ) and electrosan head... and want to add a chartplotter, radar, tv and laptop, you really need to do an electrical budget to figure out what your average daily usage is going to be, based on what you will be using and how long on average per day you will be using it. You then have to figure out how many days on average you will be going between recharging the batteries. This will give you a base number... double it to get the size of the battery bank you'll need.
For instance, if your daily electrical budget is 120 amp-hours, and you want to go two days between re-charging the batteries, you would probably want a 500-600 amp-hour battery bank.
As for equipment.... I would highly recommend getting a Xantrex inverter/charger, like the Freedom 20, which would give you 2000 Watts of AC power as well as a three-stage intelligent 100 amp battery charger IIRC. I would also add a Blue Sea Dual Circuit Plus Battery
switch with the battery combiner. This would automatically combine the house and starting banks while charging dockside or under power and isolate the starting bank from the house bank, yet allow you to combine the house and starting banks to start the engine. It also would protect the electronics from the voltage drop caused by starting the engine unless you have the banks combined.
For batteries, I would recommend going with AGM batteries. This is mainly due to their higher charge acceptance rate and lower maintenance. Provided you keep the charged on a regular basis, they'll probably last as long as wet cells, if not a bit longer.
If you have any specific questions, let me know.