A Deepseascale is an item that evolves Clamperl into Gorebyss if Clamperl is traded while holding it. It also doubles Clamperl's Special Defense while held.
RSE: Give Scanner to Captain Stern in Slateport City Harbor.
DPPt: Held by wild Chinchou, Lanturn, and Relicanth.
HGSS: Route 20 (Dowsing MCHN), held by wild Chinchou, Lanturn, and Relicanth.
PBR: Buy (14,400 Poké Coupons).
BW: Route 13, held by wild Blue-Striped Basculin, Chinchou, Lanturn, and Relicanth.
B2W2: Black City (buy)