Transom.....son of a b!&*(

[Enforcer]

Get a long piece of steel or a straight crowbar sharpen the edge and start chiseling it out!! Most say that it won't stick to wood.
But if you get 95% I think you will be fine.

Also did you check the ends of your stringers?? they rot right were they go into the transom, easy to cut off the rot now add new wood and then pour , you be good forever after that