User blog comment:Lord Loss/Monster Hunter Theory... Blog/@comment-5020152-20131018191930/@comment-17735225-20131019144629

... I don't think water turning red is due to blood of creatures, but if it's capcom's explaination then I'll have to go with it.

For me I think the red colouration comes from 'the red tide effect', which is implicated to be caused by the rise of water's temperature. It is algal bloom caused by dinoflagellates (red algae), and is harmful because the algae produce toxin and use up a lot of oxygen in the water.

You can google it for more information.

But in both case, a hunter shouldn't be able to fight Dire underwater in any way. Except the toxin in this case isn't really potent enough to do much harm, which is more reasonable than swimming in boiling water filled with blood.