You’re quoting 1 Peter 3 here; it seems that you are suggesting that the immediate effect of baptism is salvation. I would suggest that it’s necessary to be careful in how you interpret this verse, both by looking at it in context and by examining what the writer means by ‘save’ here.
First, the context: 1 Peter 3:20-21 says “in the days of Noah during the building of the ark, in which few persons, eight in all, were saved through water. This prefigured baptism, which saves you now.” Notice that what is being asserted here is a comparison: in the days of Noah, people were ‘saved through water’ (διεσώθησαν δι’ ὕδατος – ‘diesothaysan di(a) hudatos’), which we see as ‘prefigur[ing] baptism’ which ‘now saves us’ (ἀντίτυπον νῦν σῴζει βάπτισμα – ‘antitypon nun sozei baptisma’). This isn’t just a statement about baptism; it’s a comparison between the rescue from the waters of the Flood, and baptism (which that rescue prefigured).
So, what does the writer mean by ‘saved’, here? Is he suggesting ‘salvation’, as we use it in a theological context? I think not. The words he uses here, in describing the situation during the Flood and the situation of baptism, are διεσώθησαν and σῴζει, respectively. These are both verbs, and both come from the same root (σῴζω, ‘to save’); they mean “to deliver out of danger” or “to deliver into safety”. Are they the same word as the one that we use theologically to mean ‘salvation’, though? That word, σωτηρίας (‘sotayrias’) is also used in 1 Peter, but back in chapter 1: “even though you do not see [Christ] now yet believe in him, you rejoice with an indescribable and glorious joy, as you attain the goal of [your] faith, the salvation of your souls.” Now, it’s true that this word, too, is derived from σῴζω, but here, it is not talking about the general notion of ‘rescue from danger’. In this usage, the writer is clearly talking about the theological notion of salvation through Jesus. Couldn’t these be the same idea, the notion of the ultimate salvation of our souls, though? I would suggest that they are not: in 1 Peter 3, he’s talking about being rescued from death (which is an event that happens and is done), while in 1 Peter 1, he’s talking about an on-going process – the process by which we are saved by Jesus. So, I would assert that the ‘salvation’ of 1 Peter 1 and the ‘rescuing’/‘saving’ of 1 Peter 3 are distinct notions.
How are we to understand the comparison between the Flood and baptism, then? It is not the process of the salvation of our souls – rather, in the case of ‘the days of Noah’, it’s simply the rescue from the waters of the Flood; likewise, in baptism, it’s the rescue of our souls from death through the waters of baptism. Yet, it’s not asserting that baptism is the end of the story; baptism is just the beginning, an “appeal to God for a clear conscience” – that is, a means for justification, not salvation. Baptism is not what saves us, per se, although it is the means through which we begin to hope for salvation…!
(Therefore, inasmuch as you utilized this passage to answer the question “how are Protestants saved?”, I would think that your answer (“through baptism”) isn’t entirely accurate, per se. Baptism is part of the answer, but not the whole story… )