Quiz

Alright, let's start discussing. The circumference is 2πR, and it would take R/V time to swim to the opposite side. However, the boat will reach that point in πR/4V time, which is faster. This simple solution is not viable.

Yeah.

Added 5 minutes later

Another option is to call Ment and have him arrest him :D

My phone is dead. :(

Added 15 minutes later

It should move perpendicularly to the direction of the maniac's movement.

This answer cannot be considered satisfactory because it is at least very incomplete and not definitive, and at least some explanation should be given as to what this will lead to. Furthermore, if one were to nitpick the wording of the answer, it is possible to move perpendicularly to the maniac's movement on the surface of the lake in two directions.