alexander seil wrote:I am sure it's already somewhere, but I am lazy, and so won't be bothered to find it
Meh

When the tooltip for blockades says "4 units" are required for a complete blockade, does that mean *units* or *ships* (elements)? I'm trying to scratch together some gunboats ships to blockade Cairo...
Units. A brig is as good as a blockade flotilla.
Do gunboats prevent enemy river crossings?
Yes
Does it really matter what you send to blockade and shipping boxes?
Yes
I assume that I can send those Confederate frigates from Richmond to either box, or are there restrictions on ship types (to either sink US shipping, or trade through the blockade box)?
What the CSA sends to a blocakde box will attempt to trade, while what the CSA sends to the Atlantic shipping box will attempt to interdict Union shipping there. Brigs (and to a much lesser extent, transport ships, I gather) are best for trade and frigates are better for raiding, but there are no prohibitions what you use where (though you as the CSA want ships with tranport capacity in the blockade boxes)
What are engineers and naval engineers good for?
Naval engineers speed up the repair and building of ships; engineers speed up railroad repairs and entenching, I believe.
Can river traffic (the one that uses your riverine shipping capacity) be interdicted by enemy coastal batteries?
If that's what they are passing on their way to their destination, then yes.
What *are* coastal batteries? Obviously, the coastal guns, but I understand that emplaced artillery can always interdict shipping? Do the coastal guns have to be emplaced (entrenched) as well?
The coastal guns will usually be within fortifications. If they are, they don't need to be entrenched too, in order to fire at passing fleets and stop supply shipments.
What are the requirements for building depots and forts?
Depot: 2 supply wagons; fort: 2 supply wagons and 4 artillery.
The units need to be fairly in order (i.e. not very damaged), and you need to have at least 51% control of the region to build in. I'm fairly sure you need a structure there too, but not sure if it's much more specific than that.
Is there a guide somewhere to the supply system?
Yes, the manual treats this in a fairly good way.
One question in particular - do the "general supplies" and "ammo" stocks displayed in the resource overview just the total sum of all the supply dumps you've got on map, or is there some kind of a "national" stockpile?
Total sum of stockpiles across the map.