Hi,
My wife and I dived in Costa Rica last year in the area that you are going to early August so it's about the same time of the year. It's during the rainy season so the visibility of the local area is poor. However we did see plenty of sharks. I think they were black tipped. Saw a sea turtle too. Also mantas....dozens of them. Frankly I do not recommend the local areas much but try to go to the Murcielagos(sp?) aka bat island and the Catalinas. They were featured in the August issue of Sports Diver. You can expect to sea mantas...sometimes I'm told enough to block out the sun if they swim above you. Also bull sharks. Costa Rica is famous for macro life. While there don't miss the above ground activities such as canopy tours, the Arenal volcano, Monteverde rainforest, white water rafting (excellent in the rainy season) and hot springs. I don't know too much about Bill Beard's I think the neatest thing I saw was above ground though. A HUGE IGNUANA, about 4 ft long.