Which is the only bird that can swim and not fly?

The penguin.

https://en.wikipedia.org/wiki/Penguin

I found more than one Kiwi bird.

Yes, the kiwi is a strong swimmer and has been observed crossing large rivers.

https://en.wikipedia.org/wiki/Kiwi

Can Kiwi fly?

No, they cannot. New Zealand prior to Moriori and Maori settlement had no predatory mammals so the Kiwi did not need to fly.
The kiwi is a flightless nocturnal bird native to New Zealand. It evolved to be flightless because it had few predators and no need for flight in pre-human-occupied New Zealand, where there were no mammalian predators, and the few bird predators large enough to threaten it were only active during the day.

To retain the ability to fly actually requires a sacrifice of strength and weight, so it made sense not to retain this unneeded ability.

http://wiki.answers.com/Q/Can_Kiwi_fly

And I have found another flightless bird that can swim!

Cassowaries can run up to 50 km/h (31 mph) through the dense forest. They can jump up to 1.5 metres (4.9 ft)[citation needed]and they are good swimmers, crossing wide rivers and swimming in the sea as well.

https://en.wikipedia.org/wiki/Cassowary

Also both the flightless huge ostriches and rheas are good swimmers.

http://usatoday30.usatoday.com/tech/columnist/aprilholladay/2007-04-23-ostriches-swim_N.htm

There are more. Here's a list of eight birds that can swim but not fly.

https://www.britannica.com/list/8-birds-that-cant-fly