Ah!Slingsby beat me to it.
More often than not a few entrepreneurs thought of this but the biggest obstacle was cost & viability.The rivers in the interior of Sabah & Sarawak are shallow & full of rapids,not much help unless you have V/TOL a/c.
MH provides a "National Service" by charging the equivalent of about USD12 for a trip to the interior.......not much money, but it's a necessity for the inland natives to get in touch with civilisation (or is it the other way around?)
Staging an airboat service in Peninsular might be more rewarding but needs marketing savvy & critical mass........but that's another story.......
Safe Flying All.
PILOST