I recommend using Corrosion X on the receiver and servos, basically anything eletrical except for ESC (you can probably skip the servos if you like) and conformal coating on the ESC, or bag and seal the ESC as best you can. The ESC needs more protection than Corrosion X offers. These methods will take care of the water concerns.
On my maiden flight I managed to flip it upside down in the water. Not a big deal, I was still able to run the motor a bit to move the plane closer to shore, but then my ESC got wet, and that killed it. I had to wait for the wind to push it to the other side. That is when we started selling conformal coating.
As far as the build goes:
Should hot glue/VHB tape the servos in place? Or is friction-fit sufficient?
Yes, glue or tape them. They will pop out if not secured.
Is it good practice to glue the motor mount in place? What about the outrigger floats?
Should not need to do anything special here. I left the floats unglued as they are a tight fit. If you want to fly the plane from grass you will want to remove the floats as they will cause the plane to spin out a lot.
Any value in reinforcing the motor mount with e.g. a carbon tube?
Not needed.
Related, is the fuselage water-tight on the bottom or are there seams that I should seal up with silicone or hot glue?
Should be water tight. There is a "spine" of foam under the hatch in the nose area. We have found this loose in some of the planes, not a bad idea to glue that down if yours is loose.
The plane like most all boats will collect some water with use and abuse. It is never a bad idea to check and drain out any that may have collected. If you flip it over in the water, it will take on some water, but can not sink. If the water is deep enough you can probably run your motor and use you rudder to bring the plane back to shore. Don't run the motor much as it will be working very hard to spin the prop in the water. (Then again, the motor is water cooled when upside down. LOL) If the water is shallow or well weeded, this may not work at all.
Little trick for getting off the water - Slowly feed in the throttle and apply rudder as needed to keep the nose into the wind. Do no pull elevator, instead let the plane come "on step" first. This means, letting the hull rise up out of the water. You will know when this happens as the plane will pick up speed and just barely be skimming along the water. Now you can put in some elevator to rise off. This takes some practice and some space, I find that many electric pilots are used to lifting off in a few feet. A seaplane will take some more space to make a nice scale like takeoff. When done correctly, it is a real thing of beauty.
Here are some pics we took, we had a lot of fun with the plane getting these photos. Just decided we need to schedule another seaplane day!
When you start off, the plane will be sitting down in the water, here is a picture of the plane under power, but not fully on step yet, you can see the water rudder is still down there, and the angle of the full is not level yet: (Please ignore that I have pulled the elevator in this picture.. I learned better later that day.)
A bit later, and we can see the plane is now on step and is picking up speed. Notice the plane sits nice and level, the water rudder is out of the water, and the floats should not be touching the water either.
Here is a better angle showing the plane up on step. If you like, reduce the throttle and cruise around on the water at a good speed.
And another:
Before you know it, you are up in the air:
If you are going to operate from grass, I suggest removing th floats and the water rudder. Some people like to keep the floats on, but for sure remove the water rudder!!
For landing, I have found that this plane is very easy to land, I pretty much turn off the motor as seen in the photo above, and glide it to the ground, when a few feet above the ground (water) start flaring and she touches down very nicely. Use throttle as needed to maintain your approach.
Hope this helps some. She really is a rewarding plane to fly.
