here, and a more recent assessment by John Sha et al. here. For general information on primate behaviour and ecology you can check out the detailed factsheets at the University of Wisconsin's PrimateInfo site.
Few primates are as closely associated with water, both fresh and salt, as the Proboscis Monkey. Proboscis Monkeys are excellent swimmers; I have seen them plunge deliberately into the water from a considerable height. Though they range widely in the forest by day, according to Boonratana and Sharma they return to the riverside for the night. An early or late boat trip should pass most or all of the family groups in the area.
Family troops are common along the Kinabatangan. Each contains a harem of females, several young at various stages of growth, and one big old male with pot belly and full banana nose.
The pot belly is not a sign of obesity or poor shape. Proboscis are leaf eaters, and like all leaf-eating animals they need to eat a huge amount up get the nutrition they need, and a place to store their food while their intestinal flora disposes of the masses of cellulose that no animal can break down on its own.
The function of the remarkable nose is a bit tougher to explain, but it may act as a sound resonator (though there is apparently no clear evidence to support this). Adult males produce a remarkable and quite un-monkey-like honking noise. There is a nice piece of video, including calls, here.
This is, I admit, no more than a "record shot", but for me it was quite a record. It shows, in a distant tree on the main river, a mother Bornean Orangutan (Pongo pygmaeus).
There are far easier places to see an orangutan than the Kinabatangan - Sepilok in Sabah or Semenggoh in Sarawak, for example - but the animals there are semi-tame, either rescued waifs or, perhaps, their eventual offspring. These, though, were the real thing - genuine wild animals, there by no other agency than their own. It was marvelous to see them there, not only because I had never seen a fully wild orang before, but because their presence may be as good a sign as any that the forest around them remains, at least in part, a vital ecosystem. Orangutans need a lot of space, and if the Kinabatangan forest can still support a thriving population then there may be hope for the survival of other creatures with less onerous requirements, including the other primates that live there.
Hoewver, the breakup of the forest into small patches as the oil palm plantations advance is a grave threat to the orangs, as they cannot cross from one patch to another to get food or seek shelter. What is needed - and what the Sabah government, working with NGOs and palm-oil producers, may at last be willing to provide, are reforested corridors bringing the patches back into contact. You can read about the problems, and the plans, here. I hope it happens - there is little time left.