Whisk soy sauce, mirin, the 1/2 cup water, honey, ginger and garlic in a bowl. Place tenderloins in one layer in a 5- to 6-quart oval slow cooker and pour the mixture around them.
Cover and cook on low for about 1 1/2 to 2 1/2 hours, checking after 1 hour, until each tenderloin reaches 145°F in its thickest center. Remove each piece as it is ready and rest for at least 3 minutes.
Pour cooking liquid into a saucepan and skim any surface fat. Simmer for 5 to 8 minutes until reduced to about 1 cup.
Mix cornstarch with cold water and whisk into the simmering sauce for 1 to 2 minutes until glossy and lightly thickened.
Slice pork across the grain, spoon a thin layer of teriyaki sauce over the slices and serve the remaining sauce alongside.