Distance tracking underwater
Tactile

Blow a horizontal jet of warm water from the wall at the end of the swim lane.  (Cold water might work too.  The jet should have a different temperature, compared to the rest of the pool, so the swimmer can feel it.)

Audio

Place a low power underwater speaker at the end of each swim lane.  The perceived sound volume will give swimmer a cue about how far he is from the end of the lane.

These methods aren't mutually-exclusive, obviously.

p.s.  Either way, let's call this thing Zanus-Alexeev device.