I have downloaded demo and tried to play it but
under water I can see nothing except the lights in Sam's suit. Then, when I stand up from water after
the underwater cave, I can't see anything except snowflakes. And the game goes 2x faster than normal.
My computer meets the system requirements and I have tried everything but it doesn't work.
Does anyone have the answer to this and is demo even official?

That is just about the quality you will get out of the full retail priced game. Just take a look at this forum and get an impression of how many problems Splinter Cell Double Agent had.

Whatever you do, don't buy that game for PC. If you are so desperate i will send my worthless copy to you for free just so you don't give Ubisoft the money. I never got my $50 out of it that was for sure.

I see that many people have problems with it but my friend bought the game and it works very well even he hasn't downloaded any patches. And he got worse computer than I have...

One of the issues is that Ubisoft didn't program the game for computers and especially for next gen computers. I'm willing to bet your friend has a halfway decent computer with a Pentium and an ATI video card from a couple generations ago, right?
Oddly enough the game probably has less problems with medium spec systems than it does with top-o-the-line systems. Even though they eliminated a huge customer base by making it exclusivly for Shader Model 3.

Still, it is a verry boring game, if you have an xbox, not a 360, go rent it and just play it through in a weekend.

Yeah, I have watched videos of the missions in Youtube last two days and I have played end of the game with my friend and I think that even I played it with hard it's too easy. Pandora tomorrow and chaos theory were ten times harder and more interesting than this. I have to say that they who don't have the game, be happy and wait for other games like R6V2 for example(hope that it doesn't have so much problems than SCDA has). I'm going to buy R6V2 and COD4 but first I have to get answer to this: Is R6V2 coming to Xp?