Regarding marker buoys and release mechanisms, I'm sure I remember reading, ages ago, somewhere about Alka-Selzer tablets, you know the ones that fizz up in water, being used to hold something down, eg a hatch. Water leaks in, tablet fizzes and dissolves, releasing the hatch and up goes the float. 160' of fishing line? Actually not a huge amount of space required, if it was on a reel mechanism of some kind. I know I've seen something on the web, in the last year, regarding a model finder. It was for planes as I remember, more of a home built affair, but I do remember it being sealed and shock proof. It sent out a radio 'blip' which could be picked up on the other half of the unit. It was quite neat and all home designed/built, by a flyer, I think in the US. Of course, it might not work in 25+ fathoms!
Mike