A Protestant might ask what the difference is between baptism of desire and trusting Christ alone for salvation. The answer is "not much." For example, a man could know he is about to be killed, acknowledge Jesus Christ as Lord and Savior and Redeemer in that moment and die a saved man. Only God knows all the factors involved. But it is altogether possible.
The key word above is "could." That doesn't mean that it is the normative process if one has time to be obedient to biblical commands, etc. (which include being baptized for the forgiveness of sins and regeneration). The anathemas of Trent regarding denial of baptism as a necessity for regeneration and salvation presuppose that one is able to be baptized.
In some situations (such as the hypothetical above of the man about to be killed) that is not possible. The Catholic Church does not teach that he would be damned simply because he didn't have time for someone to spit on him in the name of the Trinity (i.e., to produce "water" immediately in an emergency situation) and baptize him. Let's not get ridiculous . . .
But if a Catholic points out that the Church allows for baptism of desire in this case and others, then some will accuse the Catholic of denying the authority of Trent, or say that it is an individualized reading of that council, that can be dismissed! This is sheer nonsense, because in the canons concerning "On the Sacraments in General", right before the canons on baptism, from Session VII, we find Canon IV:
If any one saith, that the sacraments of the New Law are not necessary unto salvation, but superfluous; and that, without them, or without the desire thereof, men obtain of God, through faith alone, the grace of justification;-though all (the sacraments) are not indeed necessary for every individual; let him be anathema.This is baptism of desire, explicitly sanctioned by Trent. Case closed . . .