must admit, I am with the conservationists here.
Whilst my hair really does suffer in the salt and sun, I can't bring myself to put conditioner on prior to my dive, as it can't be good for the environment. Especially in the busy areas like Sharm or Cozumel. I think the spray in conditioner in the evening is probably the one to go with. At least the bulk of it will have worn off by your next dive, leaving the sea slick free.
It's a valid point with the sunscreen too, if it is highly water resistant, it is certainly not going to be particularly biodegradeable is it? I shall have to bear that in mind on my next trip.