is there a way to filter a list / record type field based on a value of another field on a salesorder via SS?

generally 'no' for native fields, but a typical workaround is to create a standard field that stands in for it (you can control the values of the custom field). I assume you're asking about something user facing (e.g. client script)?

correct. so we have a field that has all seaports and airports. I would like to filter the list so it only shows airports if a field called Shipment Type is changed to Air.