Mission Beach and Pacific Beach are both going to be close to SeaWorld and are both very touristy beaches, so they are perfect in terms of proximity to where you want to be. If you like really touristy stuff, Mission Beach has everything you'd want (lots of tourist shops and surfboard/bike rental places along the main drag, and even a Santa Monica Pier - without the pier - style amusement park). Pacific Beach is also very close to Sea World, is still touristy, but a little bit less so (it caters more to the frat boy on spring beak crowd. Lots of options as far as bars go). The same boardwalk that stretches along Mission Beach continues into Pacific Beach.
Ocean Beach is going to be less crowded (and less touristy/more fun in my opinion) and still offers a great beach. It's a few miles further from Sea World, but still close and rental housing should be much cheaper than either PB or MB. OB is across a small inlet from Mission Beach and is much more relaxed. There are still some touristy places and some bars, but neither predominate (where the touristy stuff basically takes over Mission Beach, and the bar scene dominates PB). Robb Field is also in OB where you can play tennis, skateboard, ride or run along the Ocean Beach Bike Path - which stretches for several miles- and just enjoy the greenspace. OB also has the famous Dog Beach if you like to hang out with furry friends. The beautiful Sunset Cliffs are also on the outskirts of OB.
So one of those three (MB, PB or OB) should do it for you. Any other beach neighborhoods are going to be either too far away from Sea World (Imperial Beach, Del Mar), too expensive (La Jolla), or both (Coronado).