Plague cannot target a nonliving creature because nonliving creatures are immune to poison, and Plague has the poison subtype.
According to the rules as written, you cannot target a Hydro Immune object with a hydro spell, which means that you cannot use water spells to remove burn conditions. I don't think that this makes any thematic sense, but that is what the rules as written say. I would like to see an update to the immunity rules that allowed this interaction, but I'm blanking on how to implement it elegantly.