There is a great post on Reddit where someone has posted the locations of various Giant Clams in Summerset.
About Giant Clams
The Giant Clams, when looted, may yield up to two of the following:
- Two new alchemy reagents: Powdered Mother of Pearl and Clam Gall
- Clam Gall is an offensively oriented reagent, while Powdered Mother of Pearl is more defensively oriented, granting new combinations of survival effects
- Fish meat, for provisioning
- Bait, including fish roe, guts, and worms
- Grease solvents for alchemical poisons
- Or, very rarely, clam-themed furnishings for your home