If you're looking to stay near the beach in San Diego, Mission Beach is a great spot and they have an amusement park there as well called Belmont Park.

For hotels close to Belmont Park, here is the best list to help you out:
Bahia Resort Hotel - This hotel is less than half a mile from Belmont Park and the beach and it's also located right on the Mission Bay.
Catamaran Resort Hotel and Spa - This is a tropical themed hotel that is a block from great restaurants, the beach and shopping.

For hotels close to Sea World, here is the  best list nearby:
Hilton San Diego Resort & Spa - This hotel is only 1 mile from Sea World.
Hyatt Regency Mission Bay Spa & Marina - This hotel has 3 pools so it's great for families!
The Dana on Mission Bay - This is the closest hotel to Sea World and in the summer time, you can see fireworks every night.