I'm Sorry I Didn't Build You A Stronger Ship, Young Rose

If you're being literal, it was mostly the top of the bulkheads not being sealed off the doomed Titanic, or at least hastened it's trip to the bottom of the Atlantic. When the bearded freak does the CGI demo for old Rose, he shows how the seawater rose above the top of the watertight bulkheads flowing into undamaged areas. If the top of the bulkheads were sealed off (as they were in Titanic's sister ships), the Titanic would have stayed afloat long enough for rescue if not kept her afloat for salvage or repair.

If you're referring to Mr. Andrews comments directly, he was referring to his earlier comment to Rose when she questioned the paucity of lifeboats. He told her that he built her a strong ship and not to worry.

After Titanic was destined to head to the bottom, he apologized for not builder a stronger ship.

None, it was a very strong ship.