Whisk together olive oil, sesame oil, lemon juice, garlic, parsley, paprika and seafood seasoning in a large bowl.
Add shrimp and pineapple chunks, stir gently to coat and allow to sit at room temperature for 15 minutes.
Meanwhile, combine pineapple juice, shallot and balsamic vinegar in a small saucepan. Bring to a boil, reduce heat to low and simmer for 15 minutes to reduce by a third.
Remove from heat and whisk in butter, a couple chunks at a time. Season w/ salt & pepper to taste.
Remove shrimp and pineapple from marinade (discard) and thread onto skewers
Heat barbeque (or a grill pan) w/ ridges) to medium high. Grill shrimp for 4-6 minutes, turning occasionally for even grilling.
To serve, scatter watercress onto a platter and arrange shrimp on top. Drizzle with Pineapple butter sauce and sprinkle with sesame seeds.