Along the Atlantic & Pacific coastlines, if you hear speech-inversion or time-domain 'scrambling' on VHF marine channels (& really, all over the 138-174MHz spectrum) it turns out to be small & large commercial fishing fleets, using crypto & no regard for FCC licensing/channel plans so that they can speak 'privately' amongst their vessels & share info on where the fish are.
But I've never heard of such activity on the Great Lakes, and I used to live less than 100' away from the Detroit River & thus monitored the VHF Marine Band channels regularly.
From high ground along the Pacific Coast during commercial fishing season, I've heard over a dozen freqs in & outside of the VHF Marine spectrum, clear-voice & 'secure' voice, in use by these foul-mouthed, feeble-minded fishermen.
I'm glad I'm a vegetarian, because I've even heard them talk about staying out at sea for a longer time in order to catch more fish/get a better price, even though it would mean that some of the fish they'd already caught would spoil (they planned on selling them anyway).
TV shows like 'The Deadliest Catch' make these guys seem kind of heroic, but after hearing many of them conversing on HF & VHF radio over the decades, I have a different opinion.