T.R.
I love Sea World! It does have attraction rides however I loved it before those came. The shows and sea life animals are great and it is just an all around interesting and fun place. Sorry, I don't remember the cost.
I also think the beach is well worth it. It may just be the relaxation you need after 3 days at Disney. Just the gas and maybe parking is the only cost. Great for all ages. Sun, sand castles, shells, swimming, floating etc. Chances are you will see dolphins - we have every time we have been. Also there are usually different activities available if you want (some can be pricey so try bargaining if it is something you want to do) jet ski rental, water bike rental, para sailing, etc.
If it were me I would pick the beach. I have great videos from when my kids were there as toddlers and they loved it. The shells, waves and birds were so facinating to them.
Enjoy!!