As most people have stated it's a lot easier to prevent damage than fix it. I also have long hair and thanks to many helpful people here have come up with a routine I use for my hair (it is slightly time consuming, but worth it).
Before leaving for the dive I wet my hair thoroughly and braid at the nape of my neck. Braiding keeps the hair as wet as possible. In my boat bag I have a separate 'hair bag' which includes:
*spray bottle of fresh (tap) water
*brush
*Matrix Biolage Hair Conditioning Balm
*small spray bottle of leave-in conditioner
Then on the boat (I generally boat dive) I usually wait until after the briefing and then start in on my hair. I unbraid and remoisten my hair with the fresh water. Then I take a dollop (my hair is down to my waist and I use about tablespoon or so) of the hair balm and work it through my hair. I use this kind becuase it works really well and is relatively inexpesive (I only paid about $10.00 for a descent sized tube). Then, using a thick elastic (no metal) I make a ponytail at the nape of my neck, braid it tightly, and secure with another elastic. Then I place a watersports/neoprene Hair Glove (
www.hairglove.com) over the braid. I use the 12" version, which isn't on the website, however, you can call and order them over the phone. There aren't as many fun colors, but when you take what you can get. After the hair glove I put on a Dee-Rag (
www.dee-rags.com). If it is warm out the mask just goes over the Dee-Rag, if cooler my hooded vest goes over the Dee-Rag and Hair Glove and the mask over that.
After the dives (I wait until I'm done for the day, if it's just a surface interval I leave on the Dee-Rag and Hair Glove) I remove all the hair accouterments and unbraid my hair. Then I use the on-board shower to wash out my hair. Then I towel dry, spritz with leave-in conditioner, and brush. I've never had a problem brushing it out and it stays pretty smooth and manageable. For times when a shower isn't available I usually wait to take of the Dee-Rag and Hair Glove until I can get to one (or a sink in a pinch). Then just rinse, brush and go! Hope people find that helpful.