Important: Both the methods below have now been patched and will not work. Sorry!
Certain vehicles, including the aircraft carrier and yacht, are only officially available in Heists. However, fans have found methods that allow you to get them in Free Mode! Just be aware though, that since these methods are glitches, they may stop working after future patches....
This method currently only works in the PS4 version of the game. You'll need to play this custom job. Begin an invite-only session. After that, launch the custom job. Choose one round only, and invite your friends to join.
End the game (either by winning or losing) and wait until you get the option to replay it. Select that option. On the screen after you invite your friends, quit the job. Both vehicles should be in Free Mode. However, they will vanish if you go to a new lobby.
Another glitch unlocks a police uniform for Free Mode. Begin the Prison Break - Station mission. In the lobby, choose the Cop Station team. Once the objective changes to "Steal a Police Cruiser," pause, go to "Online," and select "Find New Session."
Once Free Mode loads, open up your phone, go to "Quick Jobs," and then "Play Capture." You should spawn wearing the police uniform. At the start of each round of the Capture session, open your inventory and change your character's glasses. This will save your appearance. Once the Capture mission ends and the Job screen appears, select Free Mode. You should spawn still in your police uniform. Head to a store and save it as an outfit so you can select it any time.