A simple way is to choose between a practical setup and compositing.

For a practical shot, float the paper boat in a narrow container of dark water, such as a rain gutter or similar trough, and position it so the camera angle matches a real river background. Use a shallow depth of field to blur the container edges so they disappear, then do minor retouching if needed.

If you want to shoot in an actual river, the main challenge is the boat drifting away before you can compose the shot. Attach a thin nylon fishing line and anchor it upstream so the boat stays in place; if placed carefully, the line can be hidden underwater or be nearly invisible.

If post-processing is acceptable, the easiest approach is often to photograph the boat separately and composite it into a photo of a real river using software such as Photoshop or GIMP.

In all cases, keep the camera low to the boat for a more believable perspective, and use natural-looking water movement rather than overdone effects.