Everyone says fantastic things about Cocos Island, but it sounds like you
may want a bit more experience before tackling that one. Isla Catalina on
the north-western coast is a good place to dive, and you can get to it
easily from either Playa del Coco or Playa Flamingo, and even Playa
Tamarindo (I think). The diving there is relatively easy and might get to
see some of their nicer visitors such as mantas or eagle rays. However, a
word of warning...DON'T dive with "Costa Rica Diving" in Playa Flamingo. I
dived with the owner who seemed not to care about my desire to not go into
deco, and therefore decided to ignore me completely when I wanted to ascend
to a reasonable depth for my desired profile. Not the best buddy behaviour
I've encountered, especially when I'm a paying customer.

So, provided you steer clear of that dive operator you may well have a
lovely time.

Stan