Get a show schedule off the Internet and work off of that. If there are enough shows that day then, yeah, you can just work your way around, but some days it takes planning to see them all.
If you want to do Manta, best to get there early and do that first. Same with feeding the dolphins - get there early, head straight to the dolphin pen. If you want to do both, I *think* you'd want to do Manta, then the second feeding with the dolphins, but expect to stand in line whichever is second. Be aware that you have to buy the food for the dolphins separately, and you need cash (unless you have an Annual pass, I believe).
I had a hard time back in the day trying to find tips on scheduling for Sea World.

But I think people who're determined to "see everything" just make use of the "second day free" option (you have to go to the counter and get your ticket stamped), so scheduling isn't as key. If you're a rides person, there are only three rides, so not a challenge to get them all in. Our first day schedule was totally blown because we didn't get up in time, but we still saw and did everything we'd planned on since we went a second day (just for a few hours to fill in the gaps).