Hot water is hot

This sounds like an oxymoron or a tautology or whatever: please note, hot water is very hot. 

Well, thanks for that. Hot water is hot. Big surprise. It sounds like the 'redundant adjectives are redundant' meme (which pleasingly derives from the Simpsons: Ralph says 'fun toys are fun').

But it's just a warning telling us that the water coming from the hot tap is hotter than is comfortable, and to be careful. How do we get from one to the other?

The 'hot water is hot' interpretation arises if we understand the sentence as telling us something about hot water in general, namely that it has the property of being (very) hot. Note that in this context, 'very' seems to have lost some of its meaning and all hot water is very hot, not just some of it.

The warning about the hot tap interpretation relies on us knowing that wording on signs often omits function words like 'the' and interpreting it as if it was there: 'the hot water is very hot'. Then the definite article 'the' makes the sentence about some specific, relevant hot water that is known to us; in this case, it must be the hot water in the immediate environment, which is to say the water in the hot tap. Then it means that the water in the hot tap is very hot, where 'very' has its full meaning of 'more than usually'.