I have 2 confusions in this verse .
1) Regarding god's throne/rule on water.
This is an allegorical statement to proclaim Allah's supreme control over the source of life which depends upon water.
"We made from water every living thing."(21:30)
"All living things are created from a mixture of water."(24:45)
Allah exists everywhere therefore He does not literally sit on a (physical) throne as Allah is not physical in nature.
2) that why they the peoples would say that this is clear sorcery when nothing abnormal happened in that scene ?
i am curious to know it because this verse can be used to support extra-quranic stories in which prophet did miracle.
The claim of unbelievers, "This is not but obvious magic" سِحْرٌ مُبِينٌ in the above verse, has nothing to do with any reference to extra-quranic stories in which prophet did any miracle (the prophet did not do any miracle according to Quran). Their denial of life after death is mentioned in Quran at many places. For instance, " And yet they say: "There is nothing beyond our life in this world. We die as we come to life, and nothing but time destroys us." But of this they have no knowledge whatever: they do nothing but guess.”(45:24)
Also, it was the claim of the disbelievers all along that the prophet is affected with magic which Quran very strongly refuted at a number of places. Refer to the following verses;
“when thou camest unto them with all evidence of the truth, and [when] those of them who were bent on denying the truth were saying, `This is clearly nothing but deception!” سِحْرٌ مُبِينٌ (5:110)
“And so, when the truth came to them from Us, they said, "Behold, this is clearly nothing but sorcery!" سِحْرٌ مُبِينٌ (10:76)
“But when Our light-giving messages came unto them, they said, "This is clearly [but] spellbinding deception!" سِحْرٌ مُبِينٌ (27:13)
“When Our Clear Signs are rehearsed to them, the Unbelievers say, of the Truth when it comes to them: "This is evident sorcery!" سِحْرٌ مُبِينٌ (46:7)