Mirage you say? Well a mirage would show the boat upside down, not right side up. So we can scratch mirage off of the list of possible solutions.
A very specific type of mirage, which has a specific name. Here's a blurb from a wiki page about it:
"A superior mirage can be right-side up or upside-down, depending on the distance of the true object and the temperature gradient. Often the image appears as a distorted mixture of up and down parts.
Since Earth is round, if the downward bending curvature of light rays is about the same as the
curvature of Earth, light rays can travel large distances, including from beyond the horizon. This was observed and documented in 1596, when a ship in search of the
Northeast passage became stuck in the ice at
Novaya Zemlya, above the
Arctic Circle. The Sun appeared to rise two weeks earlier than expected; the real Sun had still been below the horizon, but its light rays followed the curvature of Earth. This effect is often called a
Novaya Zemlya mirage. For every 111.12 kilometres (69.05 mi) that light rays travel parallel to Earth's surface, the Sun will appear 1° higher on the horizon. The inversion layer must have just the right temperature gradient over the whole distance to make this possible.
In the same way, ships that are so far away that they should not be visible above the geometric horizon may appear on or even above the horizon as superior mirages."
You were asking why you could see stuff that you weren't supposed to be able to see, according to the formula. I'm only offering possible answers.
Really I can't say that the boat itself is a mirage, I think Sapient is right and that it's not actually that far out there, but that oil rig....the thing is an absolute behemoth if that's 25 miles away and still looks that huge..