Marinas -Most Common 50 Amp Power Source - 125v or 125/250v?

On the east coast 125/250-50 is pretty much standard in most slips (except small ones) not 125/50 although it may still be found at some small, back water places...

I agree with this as well, 250V 50amp is common almost everywhere. I cannot even remember where I've even seen 125v/50amp. You should have no problems using a Y adapter to go from 250V/50 to 2-125v/30amps and it's very common to do that.

Generally you'll find just one 30 amp outlet only by slips for small boats. like under 26'. Almost anywhere you go you should have (2) 30 amp outlets available If one is too distant ask the dockmaster. They can generally locate another cord to get you connected. 50 amp outlets are generally at slips for boats over 42' or so. The only 50/125s I've run into were at a county marina on FIre Island, and commercial docks.

50amp/125 is often an RV setup.

Hi. I'm new to all this. If you use a Y-adapter (50 Amp, 125/250V) as described above, are you actually getting full 30Amp/120V on each leg? or does it share 50Amp across both L5-30R legs?

50 amp at 250 v. would be 100 amp at 125 v. so there is more then enough of the 250 available when splitting to 2 30's.

Err, Ahh, Ohh,,,, NO

Your 240V/50A out let uses two x 50 amp breakers for each 120V leg that make the 240V.

So, there is 50A at each 120V/30A outlet when split.
Later boats have a service breaker near the boats connect and the main panel breaker to help to still keep it a 30A system.

My dock pedestal has two 30 amp 120 v. breakers tied together feeding a 50 amp female 220 v. receptacle where the boat is plugged in and homes main breaker panel where it feeds from has a double 30 amp 220 v. breaker.
Previous boat took two 30 amp feed lines so electrician only had to change the pedestal breaker and replace the two 30 amp receptacles with the one 50 amp 220.
Breaker size is determined by size of feed wire in my case, 30 amp as each wire is #10.

Yes, that would provide 220v at 30 amp per 115v leg.

That single 50A breaker is actually dual 50s tied together side by side.
If one leg tries to trip, it brings the other leg open with it.
Un-like the separate 30s that if one would trip, the other would remain hot.

Wire size and length.
So from your house to dock is #10 or #6 wire?

It's #10 as the plans called for, the electrician ran, and the city inspector passed. For some reason a splice box was put on the back wall before the wires go into the underground conduit to the dock pedestal. I know it's 10 as many years ago I tapped into one of the two hot legs & the neutral adding a 20 amp inline fuse and then a 20 amp GFI receptacle via a short piece of # 12; probably against code for some reason, but I saw nothing wrong with it and it's been a very handy receptacle for several decades.
Also back when I had the 38, her zincs were vanishing in like 4-6 months so one year I brought in the local big name marine electric company to find the cause. After running all of their tests found nothing wrong they installed a Galvanic Isolator on the boat after its 30 amp breaker on the always plugged in line and after that zincs lasted "forever." In her 30 years here before selling, she never had any electrolysis issues and so far the diver has been reporting the single underwater zinc on the 46 is still in good shape but its only been 6 months.