Laying cable down the coastline for 20 miles @ $30,000/mi and under the ocean for 5 miles @ $42,000 = $810,000 total cost.
The hypotenuse of the right triangle formed by the 20 miles down the coast and 5 miles out to the island is about 20.62 miles. This is found by using the Pathagorean Theorem:
So, the cost of going under the ocean directly from the main station would be
So, it's $865,852 - $810,000 or $55,852 cheaper going the route of the two legs