Perhaps I missed a critical piece of information, how do you know it is a single boat with capacity for ~250 people? In June 2014 the US Navy was reported as rescuing 282 African migrants likely trying to reach Italy. In this incident there were 6 smaller boats. Large boats are expensive and conspicuous.

Mentioned incident: http://www.cnn.com/2014/06/07/world/europe/us-navy-rescue-mediterranean/