Assuming you do not know; To start, measure the distance between the inside studs. And, there should be a number stamped in the remaining metal plate. If you have an usual diver, he should know; If not, get a better diver. Remember to stay with bronze washers and nuts. Better divers know this also. Sorry to rag on divers. There are great divers who know how to service a boat, and then the wanna-bees who have not a clue. We have a great diving company working with us. I can tell you stories of others,,,,
Is it a single stud, or 2 studs? If single stud, any 1/2" bolt-on plate will work. If it's 2 studs you need to know the on center distance between studs, as Capt Ralph said. Alternatively... there are multiple plates from mfrs like Divers Dream that make plates with horizontal bolt slots that will work with studs ranging from 2" to 9" center to center.