Split Bus electrical panel
You can't put that in a new structure anyhow

While the rule of six (230.71) is not a dead letter (there are still cases in new construction where a panel lacking a main disconnect can be installed), the "split bus" type panels you speak of are no longer Code, as they violate 408.36, Exception 1:


  408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an
  overcurrent protective device having a rating not greater
  than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supply
  side of the panelboard.
  
  Exception No.1: Individual protection shall not be required for a panelboard used as service equipment with
  multiple disconnecting means in accordance with 230.71.
  In panelboards protected by three or more main circuit
  breakers or sets of fuses, the circuit breakers or sets of
  fuses shall not supply a second bus structure within the
  same panelboard assembly.


But, marine panelboards are totally different beasts

Marine electrical gear is a different creature from the household stuff you were initially thinking of, and a bit beyond the scope of this site in some cases.  I'd talk with a specialist familiar with the subtle hazards AC on a boat poses if I were in your shoes -- electric shock drowning hazards are brutal.