In any case, I agree with 5jj.
The problem with the whole countable/uncountble issue is that, in my opinion, it's all about whether the countable or uncountable form makes any sense in our current day and age. Before the invention of bottles, a statement like, 'Can I have a water with my sandwich?' would not have made any sense (if "a water" refers to a bottle of water").
At this moment, something like 'hydrogens' or 'calciums' is not widely accepted in the scientific community. I would be very surprised if in you'd find many citations in academic journals of something like "hydrogens". This doesn't mean that we will never use hydrogen uncountably, it just means that at this moment it's not widely used as an uncountable noun. There is nothing "absolute" about hydrogen being a countable noun.