Barges are not often considered effective vessels in combat. However, Marines used them as a transport vehicles to move other crafts and equipment such as speedboats, reconnaissance planes, tanks, and ATVs, and troops.
To convert a cargo ship into an effective battleship adds a new layer to ship building.
A battle barge can be seen as a combination of an aircraft carrier and a battleship. If made correctlly, the ship can go 60+ knots. Don't be afraid to try this, because I use my barge in a number or situations. Usually, you should put on 46cm guns and Mk 45 5" guns to add the shooting range and put on torpedo bombers to increase damage to enemy ships. But when your done, all of that will cost over 200,000 resources. Here is a link on how to build the battle barge.
These ships main downfall is their low speed but due to their size they are open to improvement in the form of missiles and other modern weapons. Due to the fact that these ships have a large amount of deck room and are quite slow they may meet their fate in the hands of smaller faster and just as powerful battlecruisers. For now the fate of these ships are undecided but in an update or two the tables may be turned.