How do you approach water landing in KSP 1.1.3?

[Sharpy]

Formerly, that was easy. Use the radial air intakes as water skids - 70m/s impact durability, extremely hydrophobic, decently streamlined, and on top of that, they can double as (lousy but still...) air intakes too!

Then 1.1 nerfed their impact durability to hell.

How can I build a plane that can land on, and take off from water?

[steve_v]

In stock, carefully and with low takeoff / landing speeds. There is a stock seaplane to look at.
In FAR, dunno. Water is evil and all my planes flip or loose their pontoons.

[Snark]

It's pretty easy.  The main thing is to make sure that you land at a shallow angle.  What kills parts is vertical speed at the time you touch the water; as long as you're traveling mostly horizontally, you can skim over water pretty fast without going kablooie.

There's a stock seaplane, the Osprey, included with the game.

[Vanamonde]

Yes, the stock seaplanes work quite well. Most of them were made by NathanKell, and he's a good plane-maker.  

[Claw]

As Snark said, descent angle has a lot to do with it. Also, lifting surfaces (wings and control surfaces) work pretty well on the water. So if you put a part or two low on the vessel, that will help it skim the water. That skimming mostly helps for takeoff, but can also provide some stability for landing.

If you look at the stock Mallard, there are a couple small swept wing pieces near the back of the plane. It also has some speed brakes in the same area. The wing pieces help it maintain balance, and the air brakes add drag in the water (which can help with either steering or stopping the plane).

[Diche Bach]

Oh, I'll have to try that Osprey! 

[OrbitalBuzzsaw]

I don't try water landing with planes, I do it with VTOLs

[dire]

I always cheat and add a pair of radial chutes to all my planes. At that point I can land on (almost) anything! Taking off again, not so much...