You don't necessarily have to be in the restaurant to see the divers. You can watch them from the large windows in the viewing area, but it seems like the time can pass slowly while you're waiting for them to appear.
From the time the group assembles (usually near Guest Relations near the entrance of the park), it is about 45 minutes to an hour before the divers are in the water. They get a short behind-the-scenes tour, are provided instructions on what they can and cannot do, and then have to change into their gear.
If you made your PS at about the same time as the DiveQuest reservation, that should provide you with the right amount of time to be seated, perhaps have an appetizer, and then be served your meal just about the time the divers hit the water. (You can dally a bit if you have to!! )
A funny story about this: The second time my SO did this, I waited with the wife of another diver in the viewing area. When the divers appeared, she shot an entire roll of film of her hubby. Later, she realized that the diver whose photograph she had taken so many times was NOT her husband!! And thus she had no pictures at all of DH. . . we laughed about it, but I'm sure she wished she had photos.
MORAL: Since the divers are all in gear provided by Disney and since you don't know what their dive suit will look like, make sure you have a signal or something to recognize them by!! Sometimes it's hard to recognize them when they all have the same hair color, and it's floating up around their head!! (Especially when their physique is similar, LOL !!!)