Chris2009, the Trinity is the belief that the Father, Son, and Holy Spirit are three separate, distinct Persons, yet together make one God. Ignore the water analogies, or any analogies, they all have issues. “Substance” is a philosophical term that refers to what God is, His nature. I like to say that it is the Divine “stuff” that God is, and no one else is (this is different from the Mormon view that we all are ontologically, meaning by nature, the same as God.
The Athanasian Creed did not change anything that the Nicene Creed states. The Nicene Creed states that we believe in God the Father, His Son, our Lord Jesus Christ, and in the Holy Spirit. They are “of one substance”, as I described above; they are all the Divine essence (I prefer the word essence, which is used interchangeably with substance, as well as other words). Both creeds emphasize that there are three separate Persons, though the Athanasian Creed states that more explicitly.
Jesus can be baptized, hear the Father’s voice, and see the Holy Spirit descend like a dove, because they are all three separate Persons. We also do not believe that Jesus prayed to Himself. They are three separate Persons, not the same Person. “One substance” has nothing to do with them melding together, or separating from each other and changing shape. That does sound like a strange God, and that is not the Trinity. Again, “one substance” means that each Person is the Divine “stuff”, the Divine essence, etc. The Father is fully Divine, the Son is fully Divine, and the Holy Spirit is fully Divine. One isn’t more Divine than the other (meaning, each isn’t 1/3 divine, together making up 1 divine God. each is completely Divine).
So again, the Trinity is the belief that the Father, Son, and Holy Spirit are three separate Persons, together that make one God. They are one in substance (also said “of one substance”) meaning that they all are ontologically (by nature) the same Divine “stuff”. When we say “God” sometimes we refer to just the Father, or the Trinity together, as Mormons do as well (with Godhead substituting for Trinity).