Answering your question begins with understanding what Baptism is; Baptism is an Earthly way of acknowledging a Spiritual rebirth.
From my Southern Baptist upbringing I offer my understanding of Baptism.
That having been said exactly what does baptism exemplify? The answer to that question is:
- It is actually a burial ceremony, by that what I mean is that it exemplifies the death in payment for sin which God demanded in:
Genesis 2:17 but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die."
and it is our confession that Jesus death was in place of our second death.
- it also signifies that we are now the recipient of:
John 10:27 and 28 My sheep hear My voice, and I know them, and they follow Me. 28 And I give them eternal life, and they shall never perish; neither shall anyone snatch them out of My hand.
And just as our emersion in the water is substitution for being buried in the ground; so the water is substitutionary of the grave.
It is the earthly acknowledgement of our Spiritual rebirth and the beginning of our eternal life in the Kingdom of God.
Most churches today have a built in Baptismal for two reasons:
The water temperature can be controlled which is a distinct advantage to being dunked in the river in the middle of winter.
The baptism is before the church and in the sanctuary of God.
As far as why not use other water basins I have seen baptisms take place in a cattle watering troth; and even in the Bible the baptism of the Eunuch took place in the body of water that just happened to be handy;
Acts 8:36 through 38 NKJV Now as they went down the road, they came to some water. And the eunuch said, "See, here is water. What hinders me from being baptized?" 37 Then Philip said, "If you believe with all your heart, you may." And he answered and said, "I believe that Jesus Christ is the Son of God." 38 So he commanded the chariot to stand still. And both Philip and the eunuch went down into the water, and he baptized him.
Notice that I have highlighted two very important requirements for baptism. That is what matters not the water that substitutes for the grave.
Hope this helps.