Perhaps a sound signal that's a copy of something naturally occurring, e.g. by sea animals.
That came to mind for me too.
Fleet of small scale autonomous drones? Bonus points if they're water soluble.
An array of tactile antennae/feelers?
now you’re thinking like a catfish! i like it
Entertaining the idea of a probe sticking out the front led to a realization. The problem is you're never going to stop in time if you're at speed. You need to know well in advance that something's there.