if i wish to visit ocean park either day from 03/05 - 05/05, which day should be better?

You are suggested to go to the park on May 4 to May 5.
May 1 to May 3 is a holiday in China mainland and many mainland Chinese will go to visit the city. Thus the other days are better for you to choose.

In addition, detailed weather forecast shows that that days are mainly sunny day. Don't worry about the rain. There are several indoor activities like the theater, panda house, and indoor aquarium.