I'm not familiar with the boat in question. However, most inboard jets are more efficient than outboard jets. If that is true for this boat, 115 should be enough power.

The best way to find out is to take the boat out for a test drive, with 2 or three large buddies. If the boat gets up on plane quickly, it's enough power.

The Deschutes is big, rough water. You need power and plenty of boat to run it. 18 feet is marginal, in my view, regardless of the power.
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