World Generation
Ores automatically receive submerged variants generated by the mod; vanilla ores are verified to work, and most other ores should function as well. Patches will generate in their regular positions, but after generation, the portion of the patch which is underwater will be converted to use this submerged variant.
Submerged ores cannot be brought to the surface by landfilling; land-based mining drills can use the edge of their mining area to tap into submerged ores, but it's much more effective to use seafloor drills.
Fluid resources will generate as their regular variant on the surface of the water, and floating crude oil can combine nicely with adjusted refined concrete to make impromptu "oil rigs". Cargo Ships will generate offshore oil as usual if it's installed.
New Hardware
Wire Buoy
A floating counterpart to electric poles, the Wire Buoy closely resembles a medium electric pole, but takes up a 2x2 space, with a proportionally larger area (8x8 coverage). Can't be placed on land.
Floating Inserters
When combined with a new crafting item called the flotation platform, every variety of inserter can be changed into a variant you can place on water. The mod automatically scans for inserters and attempts to generate their floating variant; non-vanilla inserters work in the vast majority of cases, but not all (see compatibility note at end).
Belts and Mining
Flotation platforms can also be used to create floating belts and seafloor drills. For the moment, these are functionally (and in the latter case visually) identical to standard transport belts and electric mining drills respectively. Drill spriting is hard.
Illumination
To keep your aquatic operations well-lit, the light buoy is also available, crafted with a lamp and some pipe. Like its terra firma counterpart, circuit integration is supported.
Other Alterations
Refined Concrete Fill
Refined concrete is now a more flexible landfill counterpart, and can be placed on (and removed from!) water tiles and certain special Space Age tiles. It can't be removed when other entities on the tile would collide with the water it leaves behind, and other tile types (stone brick, concrete) can't replace it while it's on a tile they themselves couldn't pave.
Fishing Calibration
Inserter fishing has been limited to only work with inserters capable of filtering. This can be disabled in settings, and disables the similar, more restrictive adjustment in Cargo Ships.
What It's Not
Water vehicles and long-distance logistics aren't within Dredgeworks' scope; see Dredgepack for my recommendations on accompanying mods. If there's something besides those that you think would be a good fit here, feel free to chime in.
Compatibility Note
While inserter flotation works with the vast majority of mods, a select few may encounter issues (usually due to mismatched item and deployable names). If you run into such issues, you can disable the flotation feature using a Dredgeworks config setting, and then activate the incompatible mod; if there is still an error after this or you're a mod developer looking to resolve the incompatibility, let me know in the discussion section.