You are not thinking this through, if someone pushes you unexpectedly you instantly brace yourself, that is what Superman would have done.
If you are suggesting he made himself weak because he knew the man was going to push him away from the cage then the question remains, why walk under the fucking cage in the first place.
To seem mortal/human one would assume. That was his big thing during those years, trying to fit in and not make people suspricious.
If there were no humans on the boat at all (just supes), it would be a totally different matter.
If a relative/aquaintance wanted him to help with their fishing business, he would probably drive out with the boat (where everyone could see him), and then when he would be all alone, he'd just grab a net and swim down undersea and catch all the fish by himself, without the aid of the ship.
He wouldn't walk under a cage when alone. He would probably even lift the boat and fly home with it until he got close to land
If there was a metalcage dropping down on him (when nobody could see), he'd just brace himself and the cage would bounce off of him like beepollen-covered bitcoins on Goodrum.
