Thaw shrimp, if frozen.
Shell and devein shrimp.
Halve shrimp lengthwise.
Drain pineapple, reserving liquid.
Blend reserved pineapple liquid into cornstarch; stir in soy sauce and set aside.
Preheat wok or large skillet over high heat; add oil.
Stir-fry gingerroot in hot oil for 30 seconds.
Add green, onion and stir-fry 1 minute.
Add water chestnuts; stir-fry 1 minute more.
Remove green onion and water chestnuts.
Add shrimp to hot wok or skillet; stir-fry 7 to 8 minutes or till shrimp are done.
Stir soy sauce mixture and stir into shrimp in wok or skillet.
Cook and stir till thickened and bubbly.
Stir in pineapple, green onion, and water chestnuts; cover and cook 2 minutes more.