training 3 yr old lab

I have a 3 yr old labrador that was trained as a puppy to stay in the
yard.  Then he discovered the canal right near my house and ever since
if he gets loose he runs straight to the canal and swims until I have
to jump in and get him, because he won't come back.  What would be the
best training technique to retrain him to stay in yard or is it too
late since he his now 3 yrs old?