I would guess that dam refers to a dental dam, used for portection in oral sex.  Hot water BJ I would think would be a BJ given with hot water in the mouth, or another hot liquid.

Got to be careful with that hot water, don't want to boil your sausage

I saw a provider several years ago who kept a thermos of hot coffee on the night stand.  In the middle of our exploring each other, she poured herself a cup of coffee.  She told me you're gonna love this  She then drank a little coffee and then proceeded to give me a BBBJ.  Another cup of coffee, some more BBBJ, etc.  Literally, her hot mouth on the joystick was a sensation I've never experienced again.  It was awesome!  If this is a "hot water BJ" I'd do it again in a lightning second!

Hot water BBBJ is indeed the warming of the mouth more to create sensation while wrapping the stick.
Can be done with coffee tea or simply hot water. The alternative
to switch off to is ice water. Its a fun game while blindfolded because you dont know whats coming.. ( ahmm lol )
Dams.. yea the only one I have heard of or used is the dental dam. However those suckers taste like crap.
Do yourself a favor and try seran wrap. read this in an earlier post. A newbie was concerned about oral took a roll of seran wrap so he could play with his lady companion. Lubed her with proper implements and popped it on there. She made the comment she really enjoyed it more so than she thought she would and ended up wanting to do more that involved her little asian friend.
Condoms were used there after and it was a safe very exciting time for both... : )
xo
mo

I've never done the hot bj, but have done the ice bj's they are nice.  I'll have to see if I can get some lady to do the hot cold on me with the blindfold on.  Thanks for the idea!!

Have your provider do the hot coffee then suck on an ice cube just at the lips and tip of the tounge............oh ya!!!!!!!!! Talk about a sensation!