Byron is right. You hear people comment on needing to be able to cast 100 ft to have success down here and nothing could be farther from the truth. The majority of shots that you get whether at bonefish, tarpon or permit will be in the 30-60 foot range. While these casts are not far, that certainly doesn't mean that casting isn't paramount to success.
To have success in saltwater you must be able to double haul. The fish are rarely stationary so you need to get the fly into the zone as quickly as you can with a minimum amount of false casting. The double haul is also essential for casting in the wind. The line speed that you generate is key whether the wind is in your face or coming over your shoulder.
Like Byron said, when you take an excessively long shot, if the casts misses the mark or the fish changes direction, so much valuable time is wasted trying to get reset for another better cast. Additionally, even if the cast is perfect and the fish grabs the fly, your chances of a successful hook set are small. There is so much stretch in fly line that it is hard to get adequate pressure at a great distance to set even the sharpest of hooks in a tarpon's hard mouth. Plus it is much more fun to see the eat close to the boat.
There are few things in fly fishing that can rival the site of a tarpon tracking and eating the fly. The eat can be as subtle as a spring creek trout sipping emergers or as violent as a Volkswagon landing in the water. It is what is so great about tarpon fishing.
Andy Mill and other good anglers make tarpon fishing look easy. While it is not difficult, there is certainly a learning curve that he has tackled through hundreds of days on the bow of a skiff.
All of this talk definitely has me ready for tarpon season. It is not far away and there have already been some fish moving around on the warmer days this year. Luckily the best time of the year is almost here.