Whisk together the mustards, honey, vinegar and salt and pepper in a large bowl. Slowly whisk in the olive oil until emulsified and stir in the cilantro. Let sit at room temperature until ready to use.
Heat grill to medium.
Brush potatoes with canola oil and season with salt and pepper, to taste. Grill until lightly golden brown on both sides and just cooked through, about 2 minutes per side. Add the warm potatoes to the dressing, and toss to coat. Add the lobster and the watercress, season with salt and pepper and toss to combine. Transfer to a platter and serve.