Definitely the last one - being a double negative means "I want something that belongs to you". ie wrong and not communicating what you mean.
But it is now so widely accepted that, although technically wrong, it is colloquially common parlance and therefore totally acceptable to all except crusty grammarians who should be extinct by now.
"I think him dead" is okay too, say, in Jamaican patois but, in frightfully proper English grammar, it would only be correct if you could actively use the power of thinking to render someone dead.
ie - telepathy or shamanism - to 'think' someone dead could be just like 'shooting' someone dead.
"A bit of water" is odd because a 'bit' is usually a piece of a solid amount - but "We can expect a bit of rain" is perfectly acceptable so why not a 'bit of water'?
"Were I you..." is that lovely pest the subjunctive case which I have never seen much use for in real life when you can say "If I was you etc" - Not so grammatically elegant but everyone knows what you mean anyway. So who cares...except grammarians flexing their quill pens?