Yamaha motors are excellent and you will be pleased with them. I have had no experience with Yamaha jets but I have tons of experience with other brands such as Mercs and OMC that were used on inflatables. I have used jets from 20 horse up to 70 horsepower. I have used them on Zodiacs such as the Fastroller with speed tubes and the F470 SEAL boat as well as large and small Archilles boats. They are inefficient as Nemrod pointed out. My 70 jet was based on the HP at the jet, not on the engine. The powerhead was a 115 as I recall. Quite a considerable power drop. Something to consider is that the height of the intake to the water is critical. Most inflatables flex as they move and the water avaible to the intake can change, causing it to suck air or go too deep and make excessive spray, which slows down the boat. Adding dive gear to the boat also changes the trim. The only way to deal with this is to add a jackplate so you can dial in the proper height. On larger jets I would recommend power trim so you can instantly adjust to changing conditions. I used jets to patrol whitewater areas and the only motor I could use was a jet. They will take you anywhere you want to go and also to places you have no business being in.
If you are looking for a jet to run rapids, etc. be prepared to hit a lot of rocks.
Also be prepared to shell out $500 for a new intake boot when you hit one hard enough, which you will. If you have a power trim/tilt it won't kick the lower unit up when you hit something and normally you will get damage. Also if you are in an area with seagrass, litter, gravel or autumn leaves floating on the water be prepared to spend a lot of time cleaning out the intake. It can be a major pita. I love what a jet can do, but if I did not have to have a jet and there was anyway to use a prop, I would go with the prop. Cheaper, more efficient and faster and the transom height is not critical on a prop engine. I have shimmed up jet drives to locate the "sweet spot" and it was amazing what a difference a quarter inch of height made on the operation of the boat. Once you add a person or dive gear it changes the whole way the boat handle. It may not go up on plane unless you adjust it again. It also makes a difference on water conditions. Flat water is better for jets as waves can make it suck air. Aerated water in whitewater conditions also limits it's efficiency. If you can, try one before you buy to see if it will work for you. Think twice before buying a jet as they can pose a lot of complications for you when placed on an inflatable.
