Update 19.09.2023
No development on this boat anymore, since I was busy with my new project.
you may check it out here:

https://cults3d.com/de/modell-3d/spiel/rc-raceboat-x7

Are you guys interested in a new version on the jet boat?
Let me know in the comments.

Since there were a lot of questions on how to adapt the propeller to the shaft?
here is the solution I used:

https://de.aliexpress.com/item/4000328236126.html?spm=a2g0o.productlist.main.27.57fc282dBh8xTg&algo_pvid=5912a741-7c7d-44ed-8c4c-ddd4829664d0&algo_exp_id=5912a741-7c7d-44ed-8c4c-ddd4829664d0-13&pdp_npi=4%40dis%21EUR%2110.66%219.16%21%21%2111.34%21%21%4021038edc16925337743875233eb766%2110000001345256776%21sea%21DE%210%21A&curPageLogUid=FE2jz3krzdap

Update 08.05.2020:
we used the motor:
https://hobbyking.com/de_de/propdrive-v2-series-4248-650kv-brushless-outrunner-motor.html?queryID=821f8e5965ad229ef4e011a419d1228a&objectID=59142&indexName=hbk_live_magento_de_de_products

ESC:
70 amp esc is enough

and the battery:
https://hobbyking.com/de_de/zippy-compact-3000mah-6s-20c-lipo-pack-w-xt60u-connector.html?queryID=03c93a7b7936d4de592e38b4d6628213&objectID=82607&indexName=hbk_live_magento_de_de_products

this was a quite good setup with about 15min runtime and quite good speed.
No water cooling necessary.

Update 07.05.2020:
Uploded a new propeller (2 blades) and a new rectifier. This should improve power draw a lot.
Added ne 26mm Motor mount holes

Update 13.09.2019:
added step files so you can better modify the boat according to your needs

Update 07.07.2019:
I added a new nozzle, to to reduce the motor current. first tests looked promising

you will need a 4mm shaft (about 170mm long), two ball bearing with 8mm outer diameter and 4mm inner diameter. The jet is attached with M4 screws the rest are M3.
Note that the 4 M4 screws go directly into the Jet without nuts.
All screw holes are designed that they thread itself into the plastic.
And you will need two M5 screws for the jet nozzle. The frontend and the deck have holes the align the parts with 1,75mm filament.

For assembly just glue the parts together, then sand the gaps and leave the sawdust in there. then apply glue to the gap and it will be watertight.

Any standard servo will fit. I was powering it with a 4 cell Lipo and a 2200KV inrunner. It was really fast but the big propeller needs a lot of torque. so maybe you want to use a outrunner motor.

to get the deck watertight i used duct tape.

Also check out my Instagram for other projects (same user name)

Since this boat has no name yet, if you have an idea, write it in the comments.

So the first tests are done with a very powerful setup. You can se a short video of it on my Instagram.
It uses a 2200KV 1:8 scale motor on 4s and it needed about 240 Amps peak.
This is way to much and is due to too much restricted flow in the jet.
So we have to redesign the propeller and the rectifier.