Designing a solarpunk world. I wanted people living within the ecological limits in every biome: arctic, tundra, jungle, sailing the sea, sailing the sky in airships, under the sea in submarines. This last one posed a massive problem because submarines are extremely power-hungry, and that's incompatible with the low-energy-use theme of the world. But I didn't want to give up the Zissou vibes by erasing the submarine-tribe.

I was stuck on this for months, then had a breakthrough when I discovered an obscure technology called underwater gliders. They are AES (actually existing submarines) that use hardly any energy, can even harvest enough from ocean thermal differences to cruise the oceans perpetually hanging out with friendly dolphins. The glider system "gives the glider the ability of renewing its onboard energy stores by harvesting environmental energy from the heat reservoir of the ocean, specifically from the temperature differences of the cold deep water and the warmer surface water (available in 80% of the world’s oceans). Ranges of 30 000 to 40 000 km, circumnavigating the world, then become conceivable."

The existing ones are small drones. But a paper with (doi:10.1007/978-3-319-16649-0_12) has a section titled 'Size effects' and bigger would be better. And the paper already linked has a whole chapter discussing scaling effects.

That was a great breakthrough because I went from "Submarines are poewr-hungry nuclear behemoths" to "Submarines use basically zero energy" and did it with proven tech. The tradeoff is that you have to glide up and down, and the floor will be at a 4° tilt a lot of the time, which could be annoying.