Playful:
* Seashell Showman
* Sandcastle Showmaster
* Tide-Turner
* Wave-Waving Wonder
More Literal:
* Coastal Puppeteer
* Sea-Side String-Puller
Humorous:
* Puppet-Master of the Marine
* The Great Sea-Creature Showman
* The Dolphin-Voiced Puppeteer
Ultimately, the best name depends on the tone you want to set!