Not sure I understand? Hydrogen is NOT being attacked, but rather is usually bound to a terminal oxygen (an alcohol). Can you clarify? Do you mean carbon being attacked?
If you draw out the poster's question, you can have the electrons of water attack the carbon or the hydrogen.
My point is it appears the poster is suggesting these are alternate pathways, a reaction occurs via one or the other. This is not correct. In order for the poster to understand these reactions, the poster needs to understand the difference. Also, I think this is only true for an SN1 reaction. I'm not aware of an SN2 reaction with water, not that it doesn't occur, just that I don't know. (Is this how 18
O-ethanol is made?)