Annoying is funny word for this situation…but it wasnt as bad…
Sorry for your loss! But I still think valuable to share the experience since I don’t think many have tried this design.
In the video you fell in and the box is attached to you and fully submerged. I think it highlights how it is subjected to more challenging conditions than the typical assist box attached to a board.
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Was it a little warming? 
Actually it wasn’t at all, except the smoke. I felt nothing. I use kite harness which is pretty thick and I guess that is why I didn’t feel anything
I tend to agree with you, I think that heat raises the air pressure in the box and when I am falling to the water it cools very fast and air pressure is reduced rapidly and sucks water in.
I am pretty sure that water did the damage, as it is clear that there is hot spot and only 4 battery cells dies the rest are OK with decent charge that fits the 20 min that I was in the water. 3.8/3.9V
I think that what might have caused it, is spaghetti of wires I had (I had the BEC burned in one of the previous trials and replacing it I didn’t wrap the wires back together) and maybe one of the wires was caught between the box and the lid and allowed the box to suck in water.
My plan
- get pelican 1170 box which has a valve
- add another battery row to reduce current on each row basically move from 10s2p to 3p
Well the valve on the Pelican 1170, if it’s a pressure equalization valve it can open for a pressure difference in either direction. That might be another point of failure allowing water back in?
The pelican ones are a gortex valve. They let vapour pass though, but not liquid water.
With my DIY boogie I removed the goretex valve and used a bolt and an oring. Never had leaks, from the valve, but I wanted to reduce the vacuum effect when rapidly cooling.
Fun fact. Goretex is expanded poly tetra fluoro ethylene. Teflon on your frying pan is the same stuff, just not expanded.
Battery waterproofing. I plan to heatshrink the battery and glue the ends. So no chance for water to penetrate battery in case it penetrates the box.
I will do that as well. However i suspect it may block heat dissipation…
works for FD2… heat shrink
How do you waterproof your cable entry points?
The green there underneath is a 3d printed part.
I will pour potting silicon such that it penetrates under the blue heatshrink on the sides, inside the hole and between the wires. Then put a tie on the wires at the root (I should have printed the part with protrusion around the wires). The glue will get thicker and I will pour more such that it covers the hole.
After drying, I will tape the whole edge such that the silicon will not get scratched.
Another option is to put first liquid tape on the wires.
Additionally I should have glued the wires inside under the plastic. But I didn’t think about it at the time.
Got an advise to put silicon on the wires because it is flexible - 3m 5200/4200/4000.
Also want to try leak-seal.
You may want to 3d print a TPU spacer structure so each wire has a gap where you can pool silicon. Otherwise water will likely pass along and between the wires.
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My BEC died on first attempt. In fact , already just by running the motor on the beach.
I had only another one which is the same. But I added two hv caps on the input, from some adapter.
Now it seems to work fine, just for first partial session.
Next time I will buy some low esr capacitors to put on the input. What happens is - spike from ESC back fires to BEC. If it find low parallel resistance on BEC input, it will be suppressed.
I’ve had the same happen before.
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I see that I’m not the only one whose Matek BECs are burning out. The worst thing is when, due to a short circuit, not only the BEC itself burns out, but also the receiver and ESC.
I had one burn out short cct killed rx 
.
You can run the rx off one of your 2nd balance lead on your battery be between 5.2 and 8.2 v which will be fine for maytech rx, need to be switched for built in battery.
See if i can find it on the website for exact voltage requirements.
Yes it can run of balance, if not directly, then through converter. But it requires to connect balance connector when switching batteries (which maybe not a big deal…)
Another option is to put linear regulator instead or before the swtiching regular in one of these configurations:
- resistor+cap
- resistor+zener+cap
- hv but fast mosfet + cap at gate
- hv but fast mosfet+resistor d=>g + zener at gate
This will dampen input pulses