I can't find the video, but on Youtube there was a clip from a old U.S. Movie Magic show that mentioned Titanic's blueprints being uncovered and used for the 1997 film (also in Tom McCluksie book "No Place For A Boy" he wrote that he helped as a advisor for the film...and prior to that helped to recover and catalogue a lot of Harland & Wolff's archives).
As to their accuracy, It's possible some of them used were the original drafts for the Olympic* (prior to all the modifications) since some areas like the Master at Arms room are shown to be an exterior cabin instead of a interior cabin.
Hope this helps.
*In their defence, Titanic: Adventure Out of Time made the same mistake as well with the Turkish Baths done in the Olympic's configuration due to lack of available plans.
I am sure Cameron did have access to blueprints of the original Titanic which allowed him to create a fairly accurate replica of the ship. He did take many liberties during the production process, for example making the main Grand Staircase much bigger than the original for filmers to freely walk around and film the interior set with ease.
Also the excessive appearance of Bostwick gates throughout the entire film. In reality, only a couple of those gates existed on the Titanic and those gates blocked access to areas where all passengers were not supposed to go to, such as storage places, and the third-class dinning saloon which was open at certain hours of the day. At least that's what her plans show. Technically, steerage passengers did have ways to escape but unfortunately evacuation in those areas was poorly managed, costing many lives that night.