The boat in the picture looks to have a grey primer, most likely (pettit etching ) which bonds with and into the gel coat. Here are a couple before and afters of boats that have been soda blasted

http://www.galeforcesodablasting.com/gallery.html

The last boat on the page is a 2006 Sea Craft(soda blasted) with a Grey-Pettit etching primer under Teal bottom paint- Where the hull is prepped correctly the primer sticks, the bottom paint is removed leaving the well bonded primer