simple proportions 45" = approx. 1/9 scale or 1/8.5
48"= exactly 1/8 scale
well, if you went to 48" I think you might as well, up the jet drives from the ones I use(29mm). I would use the one at the bottom of the page of hobbylobby's jet drives, model number JM1001. I believe if memory serves correct, someone made a 48" PBR with these units. Here's the catch you'll need serious motors, if you want to go electric, like some big hackers. Personally I'd go gas and use twin zens, however this will cost quite a bit, you could also go the nitro route, but I dont think any brushed motor can turn 40mm impellers.
If you used the smaller units like mine, it may look out of scale. The 29mm units are meant for boat up to 30," on the 32" PBR they suggest, twin 700's on 29mm drives would move it pretty nicely. here's the bottom line the jet pump must put out more thrust in kilo's then the boat weighs. I was looking this up yesterday actually and a 29mm drive can move a boat up to 7 pounds. Mind you not lightning fast at 7 pounds. So twins could potentially move a boat weighing around 14 pounds. I would say more like 10 pounds, but none the less take this into consideration.
Sorry for the string of thoughts.
You could shoot for 40" and use the 29mm drives with a better motor, like twin 700sc's. That would probably work well. That would make approx. 1/10 scale.