Yes, you're right, end of March is little too late for whale watching trip. Most of the tour operators stop operating
whale watching tours in the beginning of April and some of them stop in the end of March since the snorkelling season begins. so it is good to take the trip in mid March. From March 25th onwards, everyone get ready for snorkelling tours.
Last March we did a tour, but it took long to see some of the whales and as far as i know the perfect for time whale watching is mid February.