I think it's relatively safe to conclude that it has no use. Every ship component which can be selected during ship creation has its own stats, which can be found in the ShipItemInfo file. These components work similarly to items with players, they are "equipped" by the ships.
The Hull component adds HP, fuel, defense, speed and physical resist;
The Cannon component adds attack and attack speed;
The Bow component adds fuel;
Lastly, the 4th "bonus" component simply serves as an "equipment" I guess. There are 4 available here - the two well known Fishing Net/Mechanical Arm, and the mischievous Composite Armor/Light Armor.
None of them add any stats inside ShipItemInfo. The only place I found anything related to Fishing or Mechanical arm is that their IDs are used in Fishing and Salvage skills, which have them listed as a required "equipment" in order to be casted. (Similarly to how many other skills require certain weapons to be casted).
As for the two Armors, I haven't seen their IDs used anywhere. No skills require them to be used (including unreleased skills), no damage calculations take them into account, no stat calculations take them into account. It seems to me that they're part of the many unreleased/unfinished features in the game. There's also some other components which seem to go unused and which aren't even available to be built by the existing ships.
That's kind of where I stopped my "research" with these components. Maybe I've missed something, but I would with relative certainty go ahead and say that the ship/sea system was probably meant to be designed in a much different way (There's even an unreleased class which only has sea skills) and these components were perhaps part of something bigger... but we can only speculate.
i also want to know what it is
