I think he tracked them down and knew where they were going. He wants to give Elisa sand to cersei for killing myrcella, as the gift.

But how could he have tracked them? They're on boats. Did he sneak on, check who was there, and sneak off again...at night...without being seen...and swim back to his own boat?

@UrielF said:

But how could he have tracked them? They're on boats. Did he sneak on, check who was there, and sneak off again...at night...without being seen...and swim back to his own boat?

I doubt Varys is the only one with "little birds". It's also likely that the "main cast" characters were on the fanciest boat in the fleet.

I guess he used the same magical powers which he used to build 1,000 ships so quickly.

Or in other words: poor writing. Another plothole.

I was wondering the same thing..Unless he had different fleets at different points of the sea supervising ? Maybe theres a rat on team Kaleesi

http://www.funnyjunk.com/A+got+maps+compilation+pt1/funny-pictures/5288951/

This map shows the 'narrow seas' . Looks like 1500miiles by 300-600miles wide. That's a sea area ranging from ~ 1/2 million to one million miles. Even if the area of interest is 1/10th of this , that's 45,000-90,000 mile^2. Those sailing ships can manage 8 knots and the sweep width in poor seas/weather is about five miles.

Maybe they can sweep 450-500 miles² per hour , which means it would take them something like 90-180 hours to find this fleet. In that time the opposing fleet could travel half way around Westeros. In short the chances of these two fleets to meet is maybe 1 chance in 10. Pure luck.

maybe urin grey joy also has a fleet finder horn?