Sensing magnetic pulses from a rowing boat impeller
The way to do this is with a Hall effect sensor. I like the Allegro A1324.



It's a simple device. Give it 5v and 0v on pins 1 and 2. Pin 3 gives a voltage proportional to the magnetic field. You can sample this with an ADC, or you can use a comparator to convert this analogue value into a digital square wave.