Normally if we go to buy the ticket directly at the Underwater World Pattaya, it will cost 1 visitor 500 baht, and 300 baht for children with height below 130cm.
However, before going to the Underwater World, I was suggested by a taxi driver to contact Mrs. Pim. She opens a travel agency. The price I got to visit the Underwater World was 450 Baht per person. Together, I went with 8 people, thus I saved 400 Baht. I am quite sure that the taxi driver gets some commission. However, I didn't care as the tickets worked well and I got a discounted price.
If you contact the agent directly, I guess you could get even lower than 450 Baht per person. Give it a try!
Her address is in 336/6 M.9 Central Pattaya Road. Tel: 081 782 7676, 093 111 4407