I have been wanting to build something like this also for surf fishing. I built one out of 6 inch pvc and one of those seadoo
handheld things with the propeller it worked kinda but steering it was hard i was steering it with a fishing rod and reel.
If you make it remote control and go out 1000 yards it will be real hard to see it to steer. I just need one to go out about 300 yards i would like to have a kontiki but they are way over priced.