Hello Alessendro,
 
I just arrived in the US a few months ago.
 
At my marina, I have what they call the 125/250 Volt 50 amp - which is
1 black wire of 110 V
1 red wire of 110v
1 green wire for ground
1 white wire for (?)
 
Since I didn't want to alter my cordset, I decided to make an extension so I purchase 2 plugs and a few feet of wire.
The first one wt West Marine for 125/250 Volt 50
The second one was more difficilt to find (it is the one that will fit into my existing cosrdet)
I found it on Amazon:
HUBBELL HBL332C6W AC Connector IEC60309 332C6W IEC 309 Pin & Sleeve
http://www.amazon.com/gp/product/B004790VNG/
Cost $150
It has only 3 wire, but this is ok. 
For the wire itself, at first I purchase the big 50 amp, but it will not fit into the plug, so I had to use 30 Amp wire. 
 
So I connected:
The Black (110 V) of the 125/250 Volt 50 plug to the black of the extension cord
The Red (110 V) of the 125/250 Volt 50 plug to the white of the extension cost
The Green of the 125/250 Volt 50 plug to the green of the extension cord
 
Then at the end of the extension cord I put the
HUBBELL HBL332C6W AC Connector IEC60309 332C6W IEC 309 Pin & Sleeve
 that I mentionned above:
the black (110 V) of the extension cord goes to the brown of the Hubbel plug then boat
The white (110 V) of the extension cort goes to the blue of the Hubbel plug then the boat
The Green (Ground) of the extension cord goes to the yellow & green of the hubble plug and the boat.
 
I have been running all 3 Air conditionning, plus 30 amp charger, plus some electric outlet without any problem. 
 
You already know about the microwave, dishwasher, washer/dryer and watermaker.
 
Sincerely, Alexandre
713-412-6704
SM2K #289 NIKIMAT
Seabrook, Texas - USA