Add to the list of available constructions:

Ship (Machinery)
* Cost:
o Average Guys:
+ Initial Cost: any positive amount of population up to 25
+ Cost per Period: 8 metal, 5 grain
+ No. of Periods: 3
o Robots:
+ Initial Cost: any positive amount of population up to 25
+ Cost per Period: 8 metal, 5 petroleum
+ No. of Periods: 3
* Effects:
o When the construction of a ship is initiated, the amount of population paid for it shall be included in parentheses behind the new entry. When the ship is completed, that number shall be included in parentheses behind the given name of the ship, and that population is considered to be on the ship. If, at any time, there is no population on a ship, that ship is demolished.
o As long as a ship is in an oceanic territory, the territory is occupied by that ship’s controller, or disputed between em and any other country with a ship in the territory, if another country has a ship in the territory.
o The Controller and Population field of the world listing of an oceanic territory with one or more ships in it shall be broken out as follows:
#  Controller: Occupied by country1 OR Disputed by (Country1, Country2, etc…)
# Population: (total amount) from country1 ((population on Ship1) from Ship1, (population on Ship2) from Ship2), etc., (total amount) from country2, etc…

Change the paragraph mentioning sea transport in rule Transportation to read:

If an Oceanic Territory is used as an intervening Territory for Transport, this adds a Sea Transport to the Transportation type. Each period of Sea Transport costs half the usual Transportation rate but requires an additional cost of 10 Metal for every 10 population moved or fraction thereof unless the country performing the transport controls a ship in the Oceanic Territory.

Add to rule Conflict:

When an Oceanic Territory is disputed, a country controlling a ship in that territory may perform an assault attack. In eir attack post, e must name a ship e controls in that territory and a ship the defending country controls in that territory. The population on the former ship is the attacker’s presence and the population on the latter ship is the defender’s presence. Any population changes are performed to the amount of population on the ship.

Just a random idea. More attacks could be added later.