1)
Why does it say "Sabuni cha'Mayim"?
1.
Radak: Just like water surrounds the world (the inhabited land), so fear of You surrounds me.
2)
Is "Hikifu" the same as "Sabuni"?
1.
Radak: Yes. The matter is repeated in different words, to strengthen it.
2.
Malbim: No. Hakafah is greater; it is surrounding on every side.
3)
Why does it say "Yachad"?
1.
Malbim: They did not surround me one after the other, rather, all of them at once.