[capt vimes]

Well - i have to disagree with desert rat...
The usual stuff you get are all using the remaining light and amplifying it...
Good if you have some light like a full moon but not so good, if it is i.e. only cloudy...
The IR equipment works no matter how much light there is, during daylight even and the resolution and sensitivity of those has just become incredible in the last couple of years...
They fully automatic systems with warnings and whatnot were initially developed for commercial shipping to detect icebergs:
FLIR Systems | Thermal Imaging, Night Vision and Infrared Camera Systems

 

[capt vimes]

Tdw - european waters - especially the med - are awful... To much of that stuff around, although there is nothing more left to fish...

Regarding the IR - the sensitivity is said to be around a temp difference of about 0.1 degrees... That should be enough to detect buoys, lobster pots and stuff... Anything else like small boats do show up on those displays like flashlights...
But they can be blanketed like any other ray detection system...
I do have some (little) experience with IR systems and the best blanket is plants, well of course with leaves on...
We could not see a guy standing behind a bush from 15 meters distance with a system the border police is using to observe the green borders, although he was clearly visible through all the gaps in between the leaves to the naked eye...

On the other hand - how many plants are out there on the sea?