The back row of the whole amphitheater (including the Fantasmic Dinner Package area, which is on the far right as you face the stage) is reserved for people using wheelchairs and/or
ECVs. This is behind the back row of benches, so some people say it is behind the back row.
The arrangement is a few empty spots for a wheelchair/
ECV to park, then a seat for one person to sit next to the guest in the wheelchair/ECV. The rest of your party will be sitting in the 2 rows ahead of the back row - sort of directly in front of the person using the wheelchair/ECV. A usual party is considered to be 6 (5 plus the person with a disability).
The show includes some some portions that are movies on screens made of water mist. There are 3 of these, so each section of the amphitheater has one that is directed toward it. The action does take place all over the stage and there are also portions that take place on boats, which start off stage at the right and going around the water to off stage at the left, so they pass by every section of the seating.
If you are not using the dinner package, you enter the area with all the other guests and stay in the regular line until about where strollers are pulled off for parking. At that point, a CM will get your party and lead you to seats (since there is only wheelchair/ECV seating at the specific areas, they make sure your party is shown where to sit).
We have been to WDW in some very busy times - including Spring break. We usually get into the line soon after the gates open for the line to start moving into the amphitheater (about 1-1.5 hours before the show starts). We have gotten there as late as 15 minutes before the show and still gotten a place and gotten there as far ahead as 45 minutes and got one of the last places. General busy-ness in the parks doesn't always translate into busy for people using wheelchairs and ECVs.
Some people use the Fantasmic Dinner Package, where you make reservations for dinner at specific restaurants and get tickets that allow you to go into the amphitheater about 30 minutes before the show begins. I personally don't feel that it's necessary. We have not had problems getting in.
Isn't there some down in front also? I remember seeing signs for them with a warning you might get wet.
There are a few accessible seats in the front row, although I don't know how many there are or how early you would need to get there. I also don't know if there are any in the Fantasmic Dinner area.
It is important to be aware that you will probably get wet from the water mist used to make the movie screens. If the water is blowing away from you, you might get a little wet. If it's blowing toward you, you will get quite wet. When we sat in the front row, we wondered why they were selling beach towels before the show started. After it began, we knew what they were for.