I live in DR and have been to Silver Banks several times . Don't have been to Tonga , vis is suppose to be better there but i think at Silver Banks the amount of whales is bigger.
I can recommend you 2 operations : aquaticadventures.com and consciousbreathadventures.com . They have the best dinghies and guides . The vessels they use are similar in terms of comfort and size.
I was there last week of march this year and had a blast
To be in the water with the whales is a magical experience . If you need further info. about Silver Banks let me know