This is an excellent question, and that is where venial and mortal sin come into play.
There is an obvious inherent weight to different sins. A white lie is not as heavy a sin as killing someone. We all agree on this. So being slightly immersed would not weigh as much (unless you are slightly immersed in sexual sin or murder or extreme theft etc). Venial sins merit purgatory, according to Catholics, which is a temporary cleansing. Once that is complete, then one would enter heaven.
Now if the "slight immersion" was a unrepentant mortal sin, such as those mentioned before, then hell awaits.
Now, I am unworthy to judge. And there are graces that God can impart at any moment, if He pleases. So maybe in a last moment, one becomes contrite and wishes to repent, but maybe lacks the time and mental capability (due to bodily failures onset at death), then I am sure God will be merciful. After all, He does love us. But for the unrepentant mortal sinner, God is infinite love, but He is also infinite mercy. Thus, chances are good they may simmer.
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