Custom ConnectionField in graphene

I'm not understanding how to use custom fields in a ConnectionField in graphene. I have something like:

class ShipConnection(Connection):
    extra = String()

    class Meta:
        node = Ship

SHIPS = ['Tug boat', 'Row boat', 'Canoe']

class Query(AbstractType):
    ships = relay.ConnectionField(ShipConnection)

    def resolve_ships(self, args, context, info):
        return ShipConnection(
            extra='Some extra text',
            edges=???
        )

Normally, you'd say:

    def resolve_ships(self, args, context, info):
        return SHIPS

but how do you return something in extra and return a list?

The proper way to do this is exactly explained here.

class Ship(graphene.ObjectType):
    ship_type = String()

    def resolve_ship_type(self, info):
         return self.ship_type

    class Meta:
          interfaces = (Node,)

class ShipConnection(Connection):
    total_count = Int() # i've found count on connections very useful! 

    def resolve_total_count(self, info):
        return get_count_of_all_ships()

    class Meta:
        node = Ship

    class Edge:
        other = String()
        def resolve_other(self, info):
            return "This is other: " + self.node.other

class Query(graphene.ObjectType):
    ships = relay.ConnectionField(ShipConnection)

    def resolve_ships(self, info):
        return get_list_of_ships_from_database_or_something_idk_its_your_implmentation()

schema = graphene.Schema(query=Query)

I don't know if this is recommended, but the resolve_total_count method can also be implemented as:

def resolve_total_count(self, info):
    return len(self.iterable)

I don't know if the iterable property is documented anywhere, but I was able to find it while investigating the Connection class