I’ve spent plenty of time over the years on or around Bangkok’s Chao Phraya River, its banks lined with historic buildings, hotels, temples, and restaurants, and its waters a thoroughfare for commuting ferries and barge traffic—so much to explore and ideal for a sundowner at a bar. But I’d never been to its final destination, where it empties into the sea in the upper Gulf of Thailand. And I certainly never expected to find whales there. A group of five friends plus my wife and I had booked a private boat with Wild Encounter Thailand. After a 40-minute drive south from the city center to the very end of the river we went a short distance east from its mouth to one of the many “klongs” or canals that branch out from its banks. There we boarded a colorful wooden boat, an open-air affair for up to 12 passengers, with …