Route out the desired path of travel in a sheet of Perspex.
Mount the Perspex 'x' inches below the surface of the water.
Attach a guide pin (dowel) to the underneath of the boat and locate the guide pin in the Perspex path
Well it sounds plausible :)
C-3PO
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Here is the original Disneyland California Jungle ride track .
You need the rear guide pin or the boat may end up just pivoting and spinning around the forward guide pin.