My best digger is my Toy (Elmo). She always ends up finding more coins than it costs to send her to dig, and has reasonable luck in finding Pearls too. On average, if I send her to dig 30 times, she will pull up at least 5 Pearls, and end up at least 20,000 coins ahead of where I started.
Her best run was a couple of weeks ago when she dug over and over for an hour. I ended up with 25 Pearls and was up 150,000 in coins. (That was a one time streak of amazing luck, but she always does well.)
Although, during the Dolphin competition, none of my fish were getting many hits on Pearls. And then only 1 or 2 at a time, instead of the 5 or 10 Pearl clusters. The Pearls seemed (interestingly) unavailable...
My next best digger is my Hippie fish. She is good at ending up on the plus side in coins, and so-so at digging up Pearls.