Rouge waves. Though I guess this isn't really a phobia as it isn't irrational.
My family is a fishing family, and my dad and brother go out into the Bering Sea to crab fish.
They have been hit by one before.
They were traveling back to St. Paul and my dad was on watch driving the boat in. It was just moderately rough with 8-10 foot seas. He then noticed a white line on the horizon, he was like "what the hell is that?". So he grabbed the binoculars and looked at it, and realized it was a massive wave. He immediately whipped the boat around to drive with the direction of the wave, and slammed the throttles down.
The wave was over 100 feet tall when it hit them, luckily it had already broken, so most of the hight was churned water. When it washed over the boat, it put 50,000 gallons of water down the exhaust stacks on the top of the boat, which are easily 40 feet off the surface of the water. This filled the engine room with water up to your waist. It also wiped any trace of anything off the deck. Picking up 3 ton sorting tables, ripping out the bolts anchoring them down and tossing them over the side.
The only thing that survived was the rope coiler, which was hanging over the side by its hydraulic lines.
We managed to get before and after pictures of the deck. (Don't have them with me)
Thankfully, none was on deck at the time, and all of the doors to the deck were closed.
Honestly to me the scariest movie I have ever seen was The Perfect Storm. As A. It's a true story, B. I can relate to the crew-members as all fishing families have made the same decision they did.