Can you estimate how much cargo a ship is transporting by measuring how tall it is compared to the sea? Is this something that would be feasible with a constellation of tandemSAR-like satellites?
Wow, that's a cool idea. I bet you absolutely can. When you image a ship broadside, it gets projected onto the surface in a flattened way we call "layover" - here's an example of how nicely ships pop out on a flat sea: https://x.com/umbraspace/status/1797970305798816030
Depends on the ship as well, not all ships change their draught ("depth") that much according to what they carry, but ships like bulk carriers and oil carriers certainly do. They report it via AIS data as well, but as its manually input it is often unreliable. An other idea that has been utilized is to look at the vake patterns (wave patterns) to estimate the speed of the ship.