You, a well-known submarinist criminal, are taken prisoner in a perfectly circular island, surrounded by water.
You stand in the center of the island, with the submarinism equipment (don't worry about its weight).
Surrounding the island there is a boat, watching you, that goes 4 times faster than you. That means that even if you go straight in the opposite direction of the boat to flee, the boat will arrive there before you.
If you manage to arrive to the water and the boat's not there, you'll manage to finally flee, since you'd go underwater.
Question is, can you flee the island? If so, how?
EDIT: There's no problem with acceleration nor delay (yes, the boat can go from full speed to full speed in the opposite direction)
Just to sum up
The island is perfectly circular and you are in the center.
The boat is always on the border of the island, moves at 4 times your speed, and can go from full speed in one direction to full speed in the opposite.
If you arrive at the border of the island and the boat is not there, you are free.