Sea vegetables (a.k.a. seaweeds)
Overfarming the soil robs vegetables of the vitamins and minerals we need. But you know what can't be overfarmed (not for a long time, anyway)? The ocean. And that's where we'll find sea vegetables, like arame, wakame, nori, dulse, and many, many others, all nutrient rich and all full of salty goodness.
Better yet, its growing space has a small footprint, so sea vegetables can feed more people with less space than earth-bound crops. That may be something worth diving into.
(For those who are curious, all sea vegetables are algae, but not all algae are sea vegetables. Got that?)