First, Marion would be dead. The Nazis would have found the ark and taken it. Would they have taken it to the island,
maybe, but we don't know where they originally planned to take it or if stealing it from the ship where Indy had it changed their initial plans.
Secondly, assuming that they would have gone to the island they would have opened it and died but no Americans or anyone else would have known the ark had been found or where it was.
The crew of the submarine would have gone looking for it after Toht and the others didn't return and the ark would have remained in Nazi custody. Which they could have then taken back to Germany for other testing.