Like Bob said, figuring out what's going on behind the panels
based on the information you give is uncertain. Also I would add that even getting information from the manufacturer may be problematic since a previous owner could have modified the original factory wiring.
A couple of comments and questions but DISCLAIMER don't start any wiring or installations based on these.
One physical breaker could be switching on/off the two 125V legs of a 250V input.
I assume your panel has a selector switch with GEN/OFF/SHORE or some similar wording that allows you to choose what source but only one that feeds the panel?
Is there a selector that allows you to select one shore power or the other?
I assume you have used your shore power cable(s) at a dock
? Does the 250V water
maker work when plugged into shore power?
Do you or have you connected both shore power cables
at once? I would guess that you should not do this unless you have a selector switch for this on the panel.
If you really want or need to know you could probably sort it out with a meter and a couple of long wires. Connect the meter to various AC loads, shore power plugs, etc and turn switches on and off to see when and where you get volts, what circuits are linked, etc. Another DISCLAIMER. Don't do this unless you know what you're doing or you could produce smoke, sparks and potentially electrified owner.