I wrote:h'okay.
First off, i highly recommend you buy a Carlon electrical box rather than build your own.... its much more reliable, easier to use, more time efficient if something goes wrong at the lake, AND unless you have the stuff laying around much cheaper!
So, if you still want to build a box let me know and i'll tell you how, but once again i recommend Carlon. you can buy them for around $5 at Home Depot, some have plastic lids too! just make sure you get MARINE LISTED (bottom of lid).
If you want a basic layout look at some of ARH's builds.
This is the bottom of the lid, make sure its marine listed
It should have a rubber seal like this
this is the inside, but really you can set it up however you want
Drill a hole in the side of the box (if one doesn't exist) and stuff it with a pvc tube. put a good bead of Electrical RTV around where the tube and box meet, on the inside and out. It must be electrical, as that is non-corrosive. Run your wires out through the pip and then fill it with the electrical RTV like so:
Okay, now... for the things outside the box. Get some rubber grip dip for hand tools.... it comes in a can like tennis balls come in... you can get diff colors and whatnot and its usually under $10.... any hardware store should have it, just ask if you cant find it.
follow the directions to either dip or brush on the stuff, just make sure you get a few good coats...nothing still opaqious.... you want it covered good. I provided in example in the kumano of one only half way done...the can had too much and it was spilling if i pushed down anymore! so now i have to brush it! Make sure you get it all the way up over the wires coming out of your batteries... otherwise its pointless!
You'll also use this for your rear servo, cover it up all the way above the seal where you can unscrew the top of the servo... however as my kumano shows, dont cover up the screws! just cover up over the line (and everything below).
If your ESC looks like this
then you are fine and dont need to waterproof it.