Logic, pure and simple: There is one Baptism, Unum Baptisma, as the creed tells us.
This one sacrament consists of water and the words (Trent)
This Sacrament is necessary for salvation because the sacraments
are necessary for salvation (also Trent)
Baptism by Desire does not consist of water and the words and is
not a sacrament. (most agree to this truth)
Ergo: Baptism by Desire does not avail to salvation