It technically opens at 9am daily. However, you want to be there by 8:00. You'll find that at 8:00 there is a long line heading down the sidewalk to get checked in. The first ones in are the first to select dolphin swim times. If you are one of the earlier ones in, you will have more swim times available and can pick something that fits your preference (early am, late am, early pm, etc). The ones who are last to arrive get whatever times are left.
So even though it opens at 9, you will likely be in the park prior to that.
Breakfast is available once you are in, and continues through a good part of the morning. Think of it as a continental breakfast.
Lunch is served for several hours. There is also nothing stopping you from eating lunch more than once. If you want to get in line and get a second meal, go ahead. I don't know what time they stop serving, but it's probably around 2:00. So no, you can't have dinner there, just a big lunch.
Snacks and drinks are available all day. There is a bar near the swimming areas, making it very easy to grab what you want. You just walk right up and take what you want. I thought it felt weird at first. After all, we're not used to being able to just take a bag of chips from a display and walk away, not having to tell anyone you are taking it. Snacks include various bags of chips, hot pretzels, maybe cookies (I don't remember). Drinks are bottled beer, lemonade, soft drinks, and a few frozen drinks. You ask for the drink and they hand it to you.
I didn't eat much in the way of snacks. I had gone through the lunch line twice. I stayed in the water until closing time (5:30). By the time I changed, cleaned out my locker, picked up photos, and bought a shirt at the gift shop, I left at 6:30. I was absolutely exhausted and took a nap at my hotel room back at Disney. Then I headed over to the parks for a bit. I'm sure at some point I ate something, but it probably wasn't until later.
You'll have to decide whether you want to do a table meal that night. If you do, you'll be locked into having to leave Discovery Cove by a certain time. You might be enjoying things so much that you want to stay until closing. And you have to factor in the time to get back to Disney, take a shower, change clothes, etc. Or you could play it by ear and see how it goes. If the evening rolls around and you are hungry enough and not exhausted, you could head out and see what is available at the restaurants (maybe somewhere like Epcot that has a lot of restaurants so you have a better chance of finding a place with availability). Or you might just decide to use a counter service meal that night.
Hope that helps.