I took a 3 day boat trip into the Sundarban National Park. It's the world's largest mangrove forest and home to the bengal tiger. It was my second trip into the Sundarbans as I went before on the Indian side.
Overall it was a bit disappointing. Chugging down the rivers with music blaring and not really close to anything to see meant we didn't see much. The few excursions onto land we did were well worn paths that see 1000's of people per day (and thus not much wildlife).
I did see deer, monkeys, monitor lizards, various birds, and some glimpses of dolphins.