We do a lot of scuba diving and snorkeling off of our TI, we live in the area, and also Key West. I have tried many small anchors advertised for kayaks, and have not found any that work on a TI. Basically any type of grapple anchor, or light weight anchor that you can throw in a bag is just not good enough for the big TI, keep in mind it 3 times the size of most kayaks. Your worst nightmare would be to dive down, then when you come back up your boat has floated away (has actually happened to us, it was quite a swim to catch it).
Of course when we are on the protected coral reefs down in KW you are not allowed to use anchors at all, you must tie to the anchor balls provided.
Our main anchor is a Guardian 4 lb G7 anchor ( http://www.westmarine.com/webapp/wcs/st ... sNum=50320
). We are very happy with the anchor, we got it at West Marine it was around $80 bucks. It's the recommended anchor for boats 17 to 22 ft long.
We do a lot of spear fishing and lobster diving down around Key West, and have never had a problem. Up here we often sail out to places like Egmont key and anchor off shore to hang out with our other boater friends. You need two anchors if your planning to do something like that, our second anchor is a folding small grapple anchor (same one tjcouch has), that works fine as the second anchor, we keep that inside the hull as a backup.
Now when you talk to the guy at West Marine, he will tell you you have to have like 8 to12 ft chain (anchor rode) on the anchor or it will not work. I didn't want that huge heavy chain chopping up my boat so I didn't get it, I said I would try it out without it first. At least in my case the anchor works just fine without it. I have a 100' 3/8 anchor rope, with another 50' stored in the hull that I can add if needed. I keep the anchor line rolled up on a spool on my motor mount. I found out that 100 feet of anchor line completely fills your boat when it is just laid on the deck (that's a lot of rope), and you need an organized way to handle the rope. I could have probably got by with 5/16 rope in retrospect, but I feel safer with the 3/8 (it can get pretty rough out in the open ocean even on a nice day.)
The hanger for the anchor is just a piece of alum square tubing that swings up to store the anchor, when we are not using the anchor it's up and out of the way back behind the boat. We have a big bow sprit and tons of huge sails and rigging up in the front of the boat so it is way more convenient just to have the anchor in the back. And if you ever have to come in through a heavy beach surf (any of the beaches around here), it's easy to just deploy the anchor just outside the surf, then slowly let anchor line out until you get to shore. Then when you go back out just pull yourself back out past the surfzone (way easier than trying to control the boat by a paddle and a prayer). However if your coming in for the day you have to walk out in waist to chest high water to go get the damn anchor.
Hope this helps