The use of water in this novel and thus its symbolism--if you can ferret it out--is not simple. On the contrary, it is complex. Water is used for washing dishes, faces, clothes. It flows under the bridge in St. Petersburg. It is for drinking. It is for soothing dying men. It falls in rain. It is the receptacle for discarded things, even lives. It is for making tea and cleaning up crime. The trick is to find a common thread between these--or at least some of these--to identify a symbol or symbols.
Some key clues seem to be these. Raskolnikov is drawn to the waters of the Neva that run under the St. Petersburg bridge. Razumihin gives Raskolnikov water and cleans him with water when he is found ill or fainting. Petrovitch makes him restorative tea at the police station. Raskolnikov tries to throw his guilt into the waters of the Neva and fails, yet Afrosinya succeeds in throwing herself, with her sorrows in her heart, into the Neva right beside Raskolnikov. And Katrina administer water to Marmeladov as he dies. One place where water doesn't have great presence, though sorely needed, is at the fire that Raskolnikov burst into to save two children:
Raskolnikov's landlady bore witness, too, that when they had lived in another house at Five Corners, Raskolnikov had rescued two little children from a house on fire and was burnt in doing so.
From these clues, it seems unfold that there are two great symbols for water. One is that it purges and cleanses as in a baptism for sin and as in the washings to reduce fever and cleanse away death's traces. This latter one (i.e., cleanse away) can be extended to the washing of Raskolnikov's ax. Cleansing may be internal as well, as restorative drinks of water and tea cleanse fever and confusion.
The second symbol may be more elusive as it is a reverse concept. Water, literally and metaphorically, saves us from burning ourselves in the struggles of life. Water here symbolizes sustenance for living: Raskolnikov was always giving his sustenance and allowance away. As a result, he began to burn with fever and thinking delirious things, literally and metaphorically.
Anxiety, cries of horror, moans... Razumihin who was standing in the doorway flew into the room, seized the sick man in his strong arms and in a moment had him on the sofa.
"It's nothing, nothing!" he cried to the mother and sister--"it's only a faint, a mere trifle! Only just now the doctor said he was much better, that he is perfectly well! Water! See, he is coming to himself, he is all right again!"