There is a good article here.
http://www.saltwatersportsman.com/saltwate...1142998,00.htmlI think 200 feet is too shallow. I would go closer to 300 feet. Zoom in on the bottom of your fishfinder. You are looking for sharp peaks and valleys...not flat. And you want there to be fish marks around those peaks and valleys. Start looking in areas where there are mounds or rounded contours looking like a hilltop or something. Basically, what you are looking for is a hard, rocky bottom, not smooth sandy. If you think you are in a good area, have an angler drop a weight down. He should be able to tell by the way the sinker is bouncing whether it's hard or soft. If it's soft and the sinker sticks, move on. A good tile area will produce bites within a minute of being in the right place. If you bring up a tile, drop a waypoint and now you own yourself your very own verified tile number. Do not publish this number anywhere on the internet unless you want that area stripped of all tiles next time you go there. They do not migrate.
Now I'm no expert as I've only been a handful of time but I think tiles are a good backup plan for when you can't find tuna. Perhaps someone with more experience than I will chime in.
Good luck.