A vehicle construction depot to be placed on the shoreline - unlocks dinghys, lvl2 for tugboat-type craft/ lvl 3 for larger offshore-tug style boats.
Have these operate similar to trucks, but on water.
They could deliver goods to shorelines - allowing easier early/mid game expansion - And construct certain undersea 'buildables', also dumping product directly on the ocean floor.
A new 'Trench Sheet' Steel-Shuttering style deep retaining wall (6 or 8 levels tall) (similar to the interlocking ones widely used globally for trenchwork and civil projects) that can be placed on the ocean floor, that can be used to construct offshore pillars or platforms.
Undersea resource veins, allowing building oil rigs.
Special undersea pipes with high investment costs for late game.
Dredgers and bulk carriers for shifting spoil and harvesting sand at the late game.
Platform mechanics / Pilings - adding expensive buildable floor tiles at configurable heights, for truly floating oil rigs. bridges?
