I hesitate to add one more opinion to this subject, but here I go. It appears to me that three different subjects are getting mixed in this discussion that should not be. The three problems are, voltage available at the pedestal under load from your boat and others, the stress on a boat's 30 amp shore power service when drawing just over 30 amps, and the proper maintenance of air conditioners. Could add a fourth as to what to expect from 208 volt three phase service when used as input to a single phase 240 volt boat, depending on the wiring configuration.
First, and for me the most important of all, is what service looks like at the pedestal when under load by your boat. SeaEric states that he is seeing a 16 volt drop (13%+), when going from no load to a 32 amp load on a 120 volt service. That is 336 watts being dissipated between the utility service and the boat electrical panel in the boat. Lots of power being lost. Far to much to blame the shore power cord/fittings and boat wiring. If it were only these items dissipating 336 watts, something would be melting. Thus the first check should be what voltage do you have on the line at the pedestal when the 32 amp load is present. This will never get better by working on the boat, the marina will have to do something to lessen this line drop in their wiring. And if the drop is in the marina wiring, loading the same wiring to 50 amps will likely result in a 26 volt drop, instead of 16.
Second, the need to have your shore power cord, and especially all connection in first class condition, can not be overstressed. When the load has been on the shore power for 10 minutes or so, take a hand held non contact temperature gun and check the temperature of the entire path. Check starting at the pedestal connection and end at the electrical connections to the air conditioner condenser. Check everything inbetween. Any variation in temperature along this path needs to be investigated, without delay.
Third, the proper maintenance of the air conditioners, I am going to leave this to the resident expert, spcoolin.
And fourth, what can you expect by converting to 240V/50A service. It does depend on the use of an isolation transformer or not on the boat, but the vast majority of conversions from 120V/30A will likely not involve an isolation transformer. So, lets look at that case. First the 50 amp shore line inlet fitting is obviously required, along with a 240V/50A shore power cord. You will also need two 50 fuse holders and fuses at the inlet for both hot wires. Next the wiring from the the new inlet to your breaker panels will need to be upgraded to tinned #6 wires, red, black, white, and green boat wire. Assuming the boat has two electric breaker panels, the likely installation is to split the two phases, one going to each panel. Red/White/Green to one panel and Black/White/Green to the other. Next it gets very specific to the breaker panel configuration, but what has to be done is to replace any wire in the panel that will be handling 50 amps to #6. If you know what you are doing in the panel this is not a difficult task, if you are not absolutely sure what needs to be done, get help. By doing this conversion, you will have 120V/50A service in both panels and will see the 120 volts on 120/240 volt service and 120/208 volt service. Just keep in mind that what voltage you will see at your breaker panels will never be any higher that what is at the pedestal under load. If the marina has good voltage at the pedestal, and it holds pretty good under load, you will now be able to take advantage of the additional power. If good voltage under load is not available at the pedestal, all your work will not accomplish any improvement.
The problem with 208 three phase service being used on boats designed for 240 single phase power through an isolation transformer has been discussed before on this forum and has to do with the isolation transformer establishing the boat neutral using a center tap on the secondary winding. I will not get into this unless someone want to start the discussion after reading all the previous postings
Pete
