Depends...

if this was prior to the building and pest inspection, yes I definitely would fix it, because the purchaser could pull out of the contract subject to the b/p clause. If it were after the b/p clause had expired, then maybe I would, maybe I wouldn't...

You are saying settlement in a few days, I'm assuming that means b/p has expired, so there is no impotus on the seller to fix your hot water system.

to murky the waters further, there is an inspection on settlement day to demonstrate the new owner is happy with the condition of the property, perhaps it needs to be brought to his attention then.

sorry this is so confusing, I hope you get some hot water soon!