If you take the word SALVATION is an all-inclusive term which describes the justification, sanctification, and glorification of the believer, then one can argue that water baptism (obedience to God's command) is "necessary" for sanctification, which is a part of salvation.

Salvation is past, present, and future:

I WAS saved from the penalty of sin (justification, past tense).

I AM BEING saved from the power of sin (sanctification, present tense). And

I WILL BE saved from the presence of sin (glorification, resurrection, the new heavens and earth, future tense).

Since learning obedience is a part of sanctification, a believer who remains unbaptized is neglecting his sanctification. In a certain sense, then, since sanctification is part of the "package" that is salvation, water baptism can be said to be "necessary" for salvation.

But salvation - all of it, past, present, and future - is an act of God upon the elect, both to will and to do His good pleasure.