Okay, so there have been a lot of rumours going around about sailing, so I looked into it.
It is completely fake.
Here is the "proof" of Sailing:
1. Now, the guy in the banner? That is a trimmed defence cape, plain and simple. Also, this image, it is an artist's impression, not the actual game! I mean, look at the staff. What is that meant to be?
2. The next bit of "evidence", is the strongest so far. The highscores picture. This looked pretty convincing, until you zoom in to 800%. Suddenly, it looks a lot different.
Notice the complete lack of blending around the sailing icon? The summoning icon is blended, so that it looks convincing at normal zoom. The sailing icon looks like a 5 minute job on paint! Also, notice around the other icons, the icons blend into the background, but not with the sailing icon.
3. Pirate implings? How on earth are they related to sailing!?
4. The new world map. This is long overdue, but how does this relate to sailing in the slightest?
5. Timings. Yes, it has been a while since the last skill, but don't you think Jagex have been busy enough with RuneScape High Detail!?
On a final note, looking at the world map, there would be no point in Sailing whatsoever, as most of the map is one block of land.