Chop the pineapple into small pieces. Put in a bowl with the pepper, chilli and coriander. Add the reserved pineapple juice and lime juice and stir well to combine. Set aside.
Heat a griddle pan to high. Brush the tuna with the oil and season well. Griddle for 1½ minutes on each side for medium or 3 minutes each side for well-done. You may need to do this in 2 batches. Serve the seared tuna steak with the salsa and the watercress.