If you want an out, you should boil the water before you drop in the shrimp but dropping in the shrimp will cause the water to temporailly to drop in temperature and stop boiling so they need to develop a good boil a second time.
Another loophole is a that “boil” is also a sensory and process description as well as an on/off state. " Smell those shrimp, they’re beginning to boil" describes what happens as the shrimp begin to cook through a boiling process. An analogous phrase would be " Smell those bakes, they’re beginning to bake."
I always imagined a different reading of the line. The “they” is the cooks, getting ready for the lunch rush, so “they’re” beginning to boil the shrimp. The “beginning” is not about the shrimp coming up to temperature. It’s about the restaurant starting its work day.