In 2012 I backpacked through Tonga. I timed my visit to be during peak whale watching time. Off the island of Vava’u from July to October boats will take you out and you can not only view humpback whales but also don snorkel and swim with them.
I had 3 days there and paid to go out the first day I was there. A long full day on the water but no whales. The captain was unable to take me out the next day as he was fully booked so I booked for day three. Day three turned out to be a replay of day one. Much to my chagrin the captain informed me that the previous day they had multiple whale sightings!
I have done enough wildlife viewing to accept that nature can be fickle. I took it all philosophically but even before leaving Tonga I was researching future whale swim options. It was then that I discovered the tiny island of Niue that also does whale swims. In fact it was the only country other than Tonga that allowed this. I “bookmarked” it for a future adventure