Swim cadence (and run cadence) are very much the same as bike cadence in that you’re measuring the number of times your arms complete strokes. The difference with swimming from cycling is that you count every individual stroke as opposed to full revolution.
Cadence is one of two variables in the swim speed equation, along with stroke length. Swim speed is equal to cadence (or stroke rate) times stroke length. Many things will affect your cadence – and stroke length – including strength, arm length, stability and mobility of your shoulders.
I’d always focus on improving your body position first before cadence, as this will reduce the amount of resistance that you’re having to overcome. This in turn may well increase your cadences anyway as the same force that you’re applying to the water with your hands and arms will be moving you faster and further.
A good stroke rate is, again, very variable depending on your physiology and stroke style. Stroke rates will vary between 55 and 90 strokes/min, and even then as an athlete looks to accelerate that cadence will change anyway. Look at it this way, the harder you push your hand back through the water, the quicker it will return to the front, so the cadence will come up without trying to ‘spin’ your arms. Just make sure there are no pauses in your stroke.