Spending most of my time diving in Northern California, I get entangled in kelp every once in a while, but as others have pointed out, its not much of a concern.
However, I did have a scary experience with entanglement while diving in Florida last winter.
I was diving the Blue Heron Bridge/Phil Fosters Park at slack tide. It was getting dark (slack tide was about 5:30pm) and I was diving Eanx 32 in an aluminum 100. Well, I started the dive late and ended up being down for a little over an hour. It was very dark when I exited the water, and I was glad I had brought along my divelight.
I had to drag around a dive flag (I hate those things) and the tide had picked up pretty severely while I was out exploring. What I didnt realize was that the line for the dive flag had gotten snagged on about fifty places on the channel wall and that the shifting water had looped the rope around me.
Visibility had dropped to about 5ft and my dive buddy was already around the corner of the wall and was swimming for shore when I realized I was fully entangled.
The current is picking up and becoming unswimmable quickly, its almost fully dark outside, vis is reduced to nothing, dive buddy oblivious, yellow nylon rope firmly wrapped all around me. And big (I mean BIG) scorpionfish lying all over the place, so I dont really want to lay down on one on accident.
Panick never set in, but I had to get my spool and slowly reel my line in untangling myself as I went and trying to get the line removed from where it had wrapped around the structure. I was not having a good time.
I was never bound up to where I could not move, but I sure was tethered to the wall and was not going anywhere untill I got untied.
I also started thinking about sharks....I have no idea why.