Imagine being the skipper of Tintin, lying in your bunk at night. You're already paranoid about electrolytic ion exchange robbing you of your aluminium hull so you've foregone copper anti fouling and regularly check your sacrificial anodes.
Now you hear a new crackling sound from beneath the hull. Whether in the marina or in a remote island anchorage, there it is again, every night, rather like the crackle of rice crispies or continuous unwrapping presents at Christmas.
Unhelpfully your crew members remark how loud it is and suggest it might be the sound of the hull dissolving.
It turns out, much to Rob's relief, that the noise comes not from the dissolving hull but from colonies of pistol shrimp. The pistol shrimp is remarkable for stunning its prey by firing a jet of water from its pistol claw at 110 kph, so fast that the pressure behind the jet drops low enough to vaporize the water behind it creating a gas bubble. As that bubble implodes it releases energy in the form of a light flash and a crack of some 200 decibels. They also use imploding bubbles of gaseous water to talk to each other, hence the continuous crackling.
Mystery solved - Rob and crew happy. Here's a fun podcast which better explains what's going on and how the US Navy used the shrimps for camouflage: