Obthomas is correct. You probably have two 30A, 120V shore power cables feeding separate busses on the main AC panel. Each buss cannot exceed 30A total (duh!) and there really is no way to obtain 220V but most boats don't need it. And as he notes there is usually a combiner switch that parallels the two busses so they are fed by one breaker, limited to 30A for both busses.
So you can plug your two 30A/120V shore power cords into two 30A/120V pedestal outlets. Another way is to get a 50A/240/120V y-splitter and plug that into the marina's 50A/240/120V outlet and then plug your two 30A/120V cords into the two y-splitter branches. Or if only one 30A/120V outlet is available plug the one in that serves the combiner switch (It will only be one of the two shore power cords. Once you flip the combiner switch the other one will be useless) and flip the switch.
Yours was a common electrical scheme back in the day which has been largely replaced today with a single 50A/240/120V shore power system. Also some marinas only have 50A/240/120V outlets so you probably need to get the y-splitter in case.
BTW, a small nit with obthomas's post. The OP does not have a "split phase" system. He has two 30A systems. They can be powered with split phase sourced power like from a y-splitter or they can both be the same phase, ie not split. The boat's wiring doesn't care which.
David