si tranducer on pontoon

Not sure how your motor mounting area is on your boat. Most seem to have some sort of box for lack of a better term. If the bottom of that box is in the water at low speeds, mounting to it may be an option. I have mine bolted to the jackplate on my bass boat and it works great, plus it is well protected there from anything I might hit while fishing or running.

Another thing to consider is if the pontoon is far enough away from the motor, the signal may shoot under the motor. The "sonar" signal from an SI transducer shoots at an angle where as traditional sonar only shoots straight down. You might rig up something temporary and try it out.

What I would do is contact a Humminbird rep or look on their website for Humminbird certified installers.
You don't have a run of the mill install with a pontoon.

If I didn't know much about these new transducers and such I would consider it prudent to pay someone who is.
A lot of folks are riding around with high dollar depth finders that work only half as good as they could. You could save yourself a lot of headache and holes you didn't need.

Ive got a 598si and never had any problems with a"blind spot" caused by the O/B. Mines on an aluminum bassboat and mounted to the hull. I would do some experimenting with it before mounting permanently.