The reverse osmosis method is not very high energy input, especially compared with distillation. It does require a relatively high inlet pressure to produce high salt rejection. There are "low energy" membranes that work tolerably well with 100 psi. I designed and built a system with these that produces about 0.6 gpm (up to 850 gpd) for about $1000, and I run it at 150-160 psi to get 99% rejection (reduction from ~350 ppm to ~2-3 ppm), and a recovery of 70% with recycle. This takes about 180W from a 1/2 HP pump, so it's about 5 Wh/gal.
It also is not that difficult with "dirty" sea water; it just needs to be prefiltered (changing sediment filters often is cheaper than changing RO membranes !), and it needs to have at least 2 stages to get the salt content low enough. I haven't built one for sea water (I live > 500 miles from the nearest salt water source), so take it with a grain of salt, but I learned a bit about it when researching the one I did build.