Bring 1 1/4 inches of water to a boil in a large straight-sided skillet. Generously season with salt. Add salmon, skin-side down; return to a boil. Remove from heat, cover, and let stand until salmon is just opaque throughout, 15 to 17 minutes. Use a slotted spatula to transfer salmon to a plate, skin-side up.
Meanwhile, bring 2 inches of water to a simmer in a large pot fitted with a steamer basket (or a metal colander). Place potatoes in basket, cover, and steam until easily pierced with the tip of a sharp knife, 12 to 15 minutes.
Whisk together buttermilk and mayonnaise in a bowl, stir in dill, and season with salt and pepper. Remove skin from salmon; flake fish into large pieces. Divide salmon, potatoes, and cucumber among plates; drizzle with dressing, and serve with lemon wedges.