Yes, it certainly has you confused, because it is a bit confusing.... Let me try to unravel it for you..
There are 50A 120V outlets which provide only 120V service. These have a certain shape plug so you only plug in a 50A 120V cord.
In addition, there are 50A 120/240V outlets which provide BOTH 120V and 240V service. The key here is that they provide BOTH voltages, not just one or the other. These outlets are also unique so you can only plug in a compatible cord.
What's interesting about the 120V/240V service is that whatever is at the other end of the cord you plug in can choose to use the 120V, 240V, or both services in any combination. And there are adapter cords that break out one of these services and make it available on a different receptacle type. For example, there are adapter cords that plug into 120V/240V service at a dock and allow you to plug in a 120V cord which would otherwise not fit the dock receptacle.
With this in mind, here's what I think you are seeing in your original posting:
Is there an electrician out there that can explain this too me? My home marina uses 50A 125V outlets for their 50A 120V shore power outlets.
If these are correct outlets then a 50A 120V/240V cord wil not plug into these outlets.
When I'm visiting other marinas most of them use 50A 125/250V outlets but they are wired to provide only 120 volts. My boat as well as most boats in the 50 foot size range require only a 120V service.
This doesn't really make sense. A properly wired 120/240V outlet should have both voltages available. Perhaps what you are seeing is the result of plugging in an adapter cable that only picks up the 120V service. Such adapter cords are made to allow 120V boats to plug into and use these more common outlets.
Now, here is the catch, at my marina they only use the 50A 125/250 volt outlets for 120/240 volt service! This means that if the majority of the boaters were to plug their 50A 125/250V cords into my marina they would end up with 240V to their boat! I find this confusing and down right dangerous to have some but not all marines use the 125/250V plugs for only a 120V service.
Hopefully my earlier explanation will help clear up this part for you. The key is that BOTH 120V and 240V is present on these outlets, and the connected boat can use either or both as it desires. There is no danger, assuming of course that everything is wired up properly. Also note that you cannot plug a 120V cord directly into a 120/240V outlet. The pin configuration is different and it will not plug in.
They make an adapter but that too is dangerous. According to NEMA (not NMEA) the industry standard for a 125/250V plug should ONLY be used for 240 volt service. My marina is in accordance with this standard.
I can't say one way or another if NEMA that a device plugged into a 120/240V outlet use the 240V service. I just don't know.
Can anyone explain to me why a marina uses this 125/250V cord at 120V?
Now I'm confused. I though you said that your Marina only allows 120V boats to use teh 120V outlets, and 240V boats to use the 240V outlets? I'm missing something here.
Not only does this not meet code but it could cause severe damage to a unsuspecting boater who thinks they are connecting to a 120V source. I have discussed this with several people and most agree that marinas that are using the 125/250V outlet at 120 volts are wired incorrectly per the NEMA standards. I purposely purchased a 125V cord but now I find that I can't connect at many public marinas like Roche Harbor and Friday Harbor in the San Juan Islands.
The plug incompatibility that you have encountered trying to hook up at Roche Harbor exists precisely to protect you against the hazard that you describe. You can't plug into the wrong service - the plugs won't go in. The only way to "convert" is via an adapter cord of some sort. Assuming everything is wired properly, you can only plug things together in a safe way.
Does this make more sense to you now?