It is a love tale and it is a tragic one. One of Thailand's most beloved Kings was Rama 5. He is credited with leading Thailand into the modern world. While he was king many of the royals would travel by boat upon the mighty rivers of the kingdom. In fact transportation by boat was so common in those days that Bangkok was known as the Venice of Asia. Even today you can take a boat to get around some of the more congested areas of Bangkok, but sadly many of the waterways have been filled in and replaced with concrete.
But back to King Rama 5. The story goes that one day his queen was traveling from Ayuttaya to Bangkok when her boat hit some choppy water. She was tossed overboard and despite the fact that she was close enough to the boat for her oarsmen to easily pull her from the waves it was a death sentence to any servant who touched a Royal. So imagine the frustration of these men as they watched their queen drowning in the river and were forced by the laws of the time to stand by and watch her go under.
In the end the King changed the rule, but his wife was lost. I really didn't get all the details, but this monument is to honor her and her tragic death. I guess the idea of questioning authority wouldn't come along for a few more generations. Hard to imagine something like this.
But don't be too sad for King Rama 5 because during his life he would have more than 50 children with 3 main wives and close to 50 different little wives. That's a lot of love!