Tonga
We dived here only a few times as we came for the Whales and you are not allowed to scuba with them. The sites we dived were from Ha'apai Island and they were ok, I couldn't say that I would dive here again to be honest, but we didn't do enough diving to be able to comment too much on it.
But, as for the snorkelling with the whales, wow, wow, WOW! I have never had such interactions with any sea creature before and it is absolutely incredible! I still have no words for what we experienced here. We got to be in the water during a heat run with about 15 males chasing one female, all of them showing of their best bubble shows or breaches or sounds. We got to play with a mum and baby all day, the baby was so curious and you could see his eye checkin us out as he swam by, with mum sitting comfortably deeper letting us play. We had a pretty close call with a cranky male who tried to drown the poor baby so he could mate with mum again. We also got up close to a singer, he was about 20m below and my whole body was vibrating, enough to make me feel like I needed to quickly use the bathroom! You could even feel it coming up through the boat! This island is much quieter than well-known Vava'u, which meant we were often the only boat in the bay, so we had all the whales to ourselves.
I noticed that you don't really need to be able to swim to do this as the guides will help you through the water, but it did affect the rest of the group as we all had to move as one through the water towards the whales, meaning we missed out on seeing a few as they had swam away by then.
There was nothing else to do on the island other than water activities, but it suited us as we did the whale swim and relax and thats all.
I would highly recommend Tonga for whale swimming and I will definitely be returning one day.