A lot depends.
Could be easy and cheap or nearly impossible to have the wireing INSIDE the wall so it all looks good and like the original part of the house..... Could do it your self if you a little handy with tools...
with the tank in the way you may have to work from the other side of the wall and cut some holes into the sheetrock and patch and paint afterwards.......
Thats even if the wall is a typical hollow stud and sheetrock wall, if its brick or solid or an outside wall of the house then working with the tank there will make it harder.....
Also this does not give you more power for more equipment because its still the same circuit and still will trip with the same breaker or fuse...I dont know how much you understand about that so ....
The drawing shows what it should look like inside a typical stud wall.....
May have to cross more studs and could be other wireing , plumbing pipes ect, already inside the wall and in the way....more than likely you will have to make the holes thru the studs....
list of stuff, should be able to get all at a HomeDepot or simular
2- 4 gang plast box
8- 15 or 20 amp recepticals ( outlets, 7 if you use the old one )
2- 4 gang recptical cover
1- 1 gang waterproof blank cover ( this goes were the old outlet is and will have a gasket )
A small box of RED wire nuts
# of feet of 12/2 romex wire with a ground ( Make sure you get enough and alow for 2 feet comeing out at each box to work the spliceing )
Im useing 4 gang size that will give you 4 receptical with 8 plugs on each side, 5 or larger size boxes are ugly IMO and harder to find.....
Spliceing is just color to color and TURN OFF POWER TO THAT OUTLET befor you open it up...
If you do it yourself and have tools then should be around $ 50. 00 or so for the stuff plus any patching and paint stuff. Every house is built different and there could be more or less wires in that outlet box......
Look it over and ask more question if you want....