Personally i have both dolphins and the secret skill is really bad.
Blue dolphin has a very bad passive if you dont have bunch of stun counter and convetors to protect otherwise your team will most likely be stunned to obvilion. So its better to keep him on fl.
Red dolphin cant be played early if you are playing sendback monsters because those that you sendback will be used for stun pluse. So you have to play red dolphin somewhere mid to late game and the chances that blue dolphin survives untill late game are low.
What i think the secret skill should be changed to if you have red dolphin in your reinforcement or on the battlefield you can cast the ability. So its actually for once a secret skill. And the same thing goes for dracorosa if you have one of thr faires in your reinforcement or on the battlefield.