Brunei does not have tour booths/shops to go to to do this your. If you would like to do a river boat tour you must go to the water front and ask the water taxi drivers if they will offer a tour to see the proboscis monkeys and fireflies. That's where you start bargaining for a fair price. We paid $B40.00 for 2 adults and 4 kids. The driver Dani was amazing.
He was kind and very helpful in finding crocodiles. The crocodiles were easy to find for us because it was low tide so, many were on the shores. We also saw otters and wild boars on the shores.
We saw a few proboscis monkeys high in the trees. We did our boat tour at night. Dani, the driver recommended that if we wanted to see them on the ground and not in the trees morning would be the best time to come. We were also there for the fireflies.
After it gets dark the bushes and trees along the shore lighten up with fireflies flickering which is also a really nice way to drive back to B.S.B in the dark. As well at dusk you can catch a beautiful sunset.
Bring bug repellent and some water. We went out from 515pm until about 715 pm. Just remember that this is just a simple boat taking you out. There were no life jackets on his boat. So these might be some things to consider when looking for a driver.