Insider Aleix Venturas reported that Rockstar Games is working on advanced water physics in GTA 6. The company has hired a team of 20 people to work on it. The insider also thinks that GTA 6 could feature surfing.
GTA 5 had very realistic water physics compared to the games of that period, so the insider's information could be true. Also, the setting of GTA 6 will be closely related to water and water vehicles.
According to rumors, GTA 6 will use the new Rage 9 graphics engine, which will try to create effects similar to WaveWorks 1.0 and 2.0 from Nvidia.