I have a 228, my livewell has two drains, one at top and one at the bottom. The bottom should be plugged when using the live well , I am adding a valve inline with the scupper to eliminate the plugging of the bottom drain. on the top I have a overflow drain that also drains out the side.
I am not sure how your boat is setup,. take a picture as I am not familiar with a 257. perhaps they are similar ??