What: December was another tough month for Oceaneering International (NYSE:OII). The company's stock sank by double digits.

So what: The main factor weighing down Oceaneering and a bevy of other energy companies last month was the price of oil. The global crude oil benchmark fell 16% for the month to just over $37 per barrel. That's well below the break-even price for many new offshore developments, some of which need oil at $75 a barrel and above to be economically feasible.

With oil prices continuing to grow weaker, oil companies are being forced to think long and hard before making a final investment decision on new oil projects. Not only that, but with oil as low as it is, many companies are cutting out any costs that aren't 100% necessary. That's leading to a significant reduction in earnings for companies working in the offshore space. Oceaneering already missed its initial fiscal 2015 earnings estimates and it expects its fiscal 2016 earnings will be even lower because its customers continue to reduce offshore activities.

This clearly isn't an Oceaneering-specific issue, with offshore peer Helix Energy Solutions Group (NYSE:HLX) offering a similarly somber view. Helix CEO Owen Kratz said in the company's last earnings report that "industry conditions continue to remain challenging and we expect Q4 results to be affected by ... a continuation of the weak industry environment." Unfortunately for Helix and Oceaneering, oil prices have only grown weaker since those comments were made, suggesting that offshore activity could be even lighter than both had initially feared.

Now what: The weaker oil prices get, the worse it will be for the offshore sector. That's why we're seeing so much pressure on Oceaneering's stock price, because it would appear that the company's initial earnings view for 2016 will again prove to be too optimistic.