Maybe I can help a little
General Waterproofing
You don't have to waterproof everything, just the sensitive electronics: ESC, servo, receiver, BEC. The power connectors, and motor don't absolutely need it.
Here's why
The conductivity of water is dependent upon the ions dissolved in it. In order of increasing conductivity, distilled water < tap water < sea water. Copper wire is about 10 million times more conductive than sea water. Water is fairly high resistance compared to a wire.
What determines damage is how sensitive your electronics are to current. It isn't true in every case, but generally, the Reciever, and ESC can handle maybe a few tens to hundreds of unwanted milliamps before frying. Depending on construction, the BEC and Servo may be somewhat more tolerant. The power connections, and motor are relatively robust. What determines current is voltage over resistance: I = V/R. Higher resistance, like that seen in water, means you need higher voltage to cause damage.
On the power connectors, getting sea water on them (which you should avoid for other reasons anyway) might cause a few microamps or milliamps to leak across, but it won't break anything. Fresh water is even less of a problem. However, chronic wetness of the connectors will eventually corrode them. It's up to you to choose to prevent now or repair later. See below on preventing corrosion.
On the motor, DC motors will run fine under water. I've done it with no waterproofing and no problems. I haven't run a sensored brushless motor submerged, but the only real difference is the hall effect sensors. Keep in mind that electricity always takes the path of least resistance. In a well-maintained motor, there will be no source of current that has no easier path to ground than through the sensor cable. Once again, your primary concern will be corrosion and dirt. You don't want either of those inside your motor. So prevent or repair.
To Prevent corrosion
Choose something that won't trap moisture. Don't use tape, hot glue, shrink wrap, etc. They will do a better job of keeping moisture in than keeping it out.
Use something that won't interfere with conductivity. For example, CorrosionX is non-conductive and bonds to metal surfaces, forming an oily film. I wouldn't use it on any connectors, but it would be fine on the motor itself for preventing corrosion, and it makes a fantastic waterproofer for the ESC, Servo board, and BEC.
I don't use anything on my motor or power connectors. I've tried dielectric grease, but it's just messy and I decided I'd rather solder new connectors every few years.
On Hot Glue
Hot glue bonds only weakly to smooth surfaces. This is what makes it desirable, because it can be easily removed for warranty replacements. But that also makes it unreliable. Eventually, that mating surface will begin to separate and may let water in.
Also, water tends to get into the smallest of openings. Even if you think you've sealed it up, any point where wires pass through is a potential weak point. Although I use silicone grease to seal up the seams on my servo, receiver box, and ESC, I generously coat the boards themselves with something water-repellent just for that one drop that might get past my outer defense system. I would not recommend coating boards with hot glue. It would trap head, and there are too many nooks and crannies that would make it a PITA to remove, completely negating its primary benefit.
If you choose hot glue, use it to seal up boxes, not coat circuit boards. Keep and eye on it, and reapply when it starts to separate. I'd recommend using it in tandem with another type of non-heat-trapping sealant to coat the boards.
That's all I have for now. Cue the angry hoards.