Some method to Dredge

It would add to the ability to shape the island to fit the factory if you had some method of mining up underwater or near-water-level terrain. It's somewhat frustrating to always have to add to the island, instead of properly being able to remove parts of the island. It could be a dredging ship (locked behind Cargo Ship technology) or some sort of fixed building (ala a reverse Stacker)

Dredging canals and rivers for ships

I agree. A dredger would be like a T2 Amphibious Excavator. Larger and only able to work on water. However it should be produced at a shipyard, not the Vehicles Depot. It would fit well with the previous suggestion of having Fishing Vessels in the game