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For a design project at school I need a way to track a short distance underwater (swimming pool). I have looked into programming an accelerometer to measure the distance traveled, but that is not accurate enough for the purpose I would like to use it for. IR and GPS are both not suitable to use for underwater purposes either because of accuracy and interference issues. I was thinking about ultrasonic waves but this seems to be an expensive solution. Any suggestion is welcome!

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The aim of the project is help visually impaired/blind swimmers. I need to track the distance between them, while swimming, and the end of the swimming pool in order to advice them when to turn because this is a big trouble for them. So, How can I measure the distance between two points in the water, one moving and one stationary, in order to sent a signal when a certain distance is achieved?

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    \$\begingroup\$ Look up something called "sonar". \$\endgroup\$
    – Olin Lathrop
    Oct 14, 2016 at 11:05
  • \$\begingroup\$ So what level of accuracy do you need? \$\endgroup\$
    – Dave Tweed
    Oct 14, 2016 at 11:14
  • \$\begingroup\$ Pretty high! I need to track a distance very close to the edge of the swimmingpool so the tolerance of the measurement should be around 10/20cm. Moreover I have the problem of lightness so the sensor (or whatever) should be very tiny. \$\endgroup\$
    – Eleonora Zanus
    Oct 14, 2016 at 15:13

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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.

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    \$\begingroup\$ This is EE.SE. I'd feel remiss if I didn't point out the option of attaching arrays of electrodes to the edge and sensing disturbances in the (electric) force. \$\endgroup\$
    – Asmyldof
    Oct 19, 2016 at 23:03

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