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Ok so I have been reading a few blogs about luffys red hawk and thought maybe I could try explain how it might work as well as how I think BH hardening works
Well the first thing I noticed when luffy used red hawk wasn't the fact his arm caught fire but oda named the attack after a f***ing amazing pistol (ruger redhawk) OK now on to the explanation

First ill start with haki hardening as we know BH is armor made from the will of the user it makes there defense stronger there attacks harder and better not forget bypassing DF powers but hardening was a big mystery well this is what I believe, the user is able to rotate the armor around a chosen body part causing friction and making the armor denser making the arm underneath said armor black

Secondly redhawk OK so when luffy uses gear second he pumps his blood around his body at high speeds making friction causing his body to heat up. Now we have a source of heat and two source's of friction

The end result when you apply two different frictions together theres ignition add something to keep the fire going (luffy) plus the added heat of gear second and we have fire

That's my reasoning for the matter but I don't know how it works underwater maybe magic idk let me know what you think