After the cutscenes, your team will be on a speedboat in the narrow underground canals with the feds hot on your trail. Hit the gas and speed away. The path is very straightforward except for that dead end around the 600m mark where you need to veer sharply to the left to avoid running aground. After that only obstacle, hit the gas and up the ramp to escape the feds in the tunnel. Another cutscene will take place.