So, perhaps I'm just blind, but I can't find the passage that Stevenson is refering to here. According to the essay, Tacitus would have been the one who described this event, which in itself is suspicious, but I can't even find a reference to Baiae in Tacitus. Suetonius does mention the mention the bridge itself and how Caligula rode across it in order to prove that he indeed could be emperor. A soothsayer employed by Tiberius had claimed that:

Caius would no more be emperor, than he would ride on horseback across the gulf of Baiae.

However, I did find a reference to the claim that Stevenson refers to in Cassius Dio. The issue here is evident. Suetonius is writing in the 1st century AD and is hardly positive to Caligula as emperor (or other emperors for that matter!) and yet he does not mention this incident. Cassius Dio is writing as late as the 3rd century AD, which is more than 200 years after the reign of Caligula. Thus it's rather safe to say that this throwing-of-guests-into-the-sea event never actually took place, but is a story that's been constructed afterwards to further emphasize how bad of an emperor Caligula was. If you're interested in reading the rest of what Suetonius has to say about this event, you can read about it here. Flavius Josephus (a Roman-Jewish author writing in the 1st century AD) also mentions this bridge, but neither he doesn't mention the special undertaking that Stevenson brings up, just the bridge itself.

As a last note I also feel obliged to comment on the claim that Caligula was deified. He actually wasn't properly deified, but decided himself that he ought to be revered as if he were a god while still alive. Ie., he deified himself, but no one else took it seriously, and stricly speaking it was the senate's right to issue state deifications and thus Caligula's deification wasn't a proper one, neither by process nor by time (deifications took place after the death of a person, not during their lifetime).