Final answer:
The question relates to the Physics subject, dealing with projectile motion. However, without more details such as height, time of fall, or initial velocity, the speed at which Matthew enters the water cannot be calculated.
Step-by-step explanation:
The question "With what speed does Matthew enter the water?" pertains to the subject of Physics, specifically involving calculations using the principles of kinematics and projectile motion. Unfortunately, without additional information about Matthew's initial velocities, the height of the cliff, or the duration of his fall, it is impossible to determine the speed at which he enters the water. Physics problems like these often require an understanding of the relationship between velocity, acceleration (due to gravity when involving a fall), time, and distance.