While I don't have any deep foundational knowledge, I do know that:

- the energy required for moving in a fluid grows sub-linear with the vessel's volume

- larger motors tend to be more efficient

- your autonomous swarm will be a gigantic waste of steel and batteries.

Not to mention the problems around the autonomous part, without which you'd also have a huge waste of manpower.

See also https://engineering.stackexchange.com/questions/38252/why-on... for some points by people I assume know a bit more than me.