We arrived in Bonaire at 5:30 am and after checking in we went through orientation and did our check-out dive. Our first real dive of the trip was a 2:00 boat dive to Petries Pillar which our DM opted to do as a drift dive due to currents. About half way through the dive we were at 50' and I looked back and saw what looked like a black school bus swimming parallel to our group and gaining fast. It took a moment to realize that I was seeing a whale shark. It just glided past us about 15' away effortlessly. Our DM has been diving here for eight years and neither he nor anyone in the dive op had ever seen one before today.