Put the potatoes in a small saucepan and cover with cold water; season with salt. Bring to a boil, then reduce the heat to medium and simmer until tender, about 5 minutes; drain.
Meanwhile, preheat a grill to medium high. Make the dressing: Combine 1/4 cup water, the cream cheese, lemon juice, horseradish, 1 tablespoon olive oil, 1/4 teaspoon salt and a few grinds of pepper in a blender; puree until smooth.
Brush the skin side of the fish with the remaining 1 teaspoon olive oil. Transfer to the grill, skin-side down; cover and cook until just opaque, about 6 minutes. Carefully transfer the fish to a plate; discard the skin. Flake the fish into large pieces.
Toss the potatoes with the escarole, romaine, radishes, apple, herbs and dressing in a large bowl. Season with salt and pepper. Add the fish and lightly toss.