Never stop moving, if you can help it! Mobility is so important.
(Outside of falling down and hitting the pavement, of course!)
I would say that if all you do is cycling, you should add some more load-bearing exercise, like weights, and a bit of running.
You should not be worried that cycling is too high impact and go for swimming for that reason (though there are great reasons to swim, and nobody should be discouraged from that!)
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1. Except for someone with a serious injury that they are treating with pool exercise, or a member of some other special population.
Lower impact being better than high impact doesn't imply that even lower impact is even better.
https://en.wikipedia.org/wiki/Bone#Remodeling
"Repeated stress, such as weight-bearing exercise or bone healing, results in the bone thickening at the points of maximum stress (Wolff's law). It has been hypothesized that this is a result of bone's piezoelectric properties, which cause bone to generate small electrical potentials under stress."
Swimming is beneficial, and is a great complement to load-bearing exercise.
Swimming, generally, is pretty poor lower-body exercise. Most swimmers have very well-developed upper-body strength, though favouring back and torso generally.
Legs get comparatively little workout, though for competitive swimmers starts (diving off the blocks) and turns (kicking off the wall) are periodic explosive strength.
Swimming can and does lead to joint problems in many cases, notably shoulders (for most strokes) and knees (breastroke or "frog" kick). The human anatomy isn't really designed for dragging itself through water, and repetitive stress injuries can emerge. The leverage effect of even a small effort exerted at the hand or foot transferred to the elbow/knee or shoulder/hip can be stunningly large.
There is the option of "aquasize" or "water walking", which can be useful as rehabilitative exercise or for the morbidly obese. This is water-assisted movement, though, not swimming per se.
Mind: so long as you're mindful of limitations, and swimming is both convenient and useful, it's a great exercise and a useful training tool, or goal. But it's possible to hurt yourself in all kinds of ways.
A useful rule of thumb is that mild soreness in muscle after exercise is generally OK. Pain in joints or connective tissue is not, and is a strong signal to back off, with total rest for days or weeks often required.
(Source: long-time competitive and recreational swimmer, some-time fitness fanatic.)
Anecdata: My knee consultant specifically banned the breaststroke kick if I go swimming (but then my knee is a disaster area and pending a replacement.)
Flutter or dolphin kick, with or without fins ("Zoomers" are particularly useful) should be safer, and actually is a reasonably good lower-body workout, particularly in conjunction with strength training (squats & deads notably).
Please note the "assumption". Be sure to ask your doctor about your specific condition, as no one here including me, has any idea what is your actual condition and what the effects would be.
Hope your knee gets better soon!
I tend to pedal slowly, though as I've gotten older my cadence has increased. Basically, find what speed works for you I think: if you're having knee pain, try increasing your cadence with a lower gear.
But it is definitely not the fast way around the track or down the road. For almost everyone, the 80-90 or 75-95 RPM cadence is most efficient. Slower will exhaust the muscles sooner, and faster (except for sprints) will squander energy on ineffective motion (as in force that doesn't get to the wheel).
Also, unless you are doing such low-RPM/Hi-torque levels that you literally fail in 6-20 strokes, you are doing closer to a bodybuilders' bulk-up routine. Going to failure in few sets builds peak strength. Doing more sets doesn't build peak strength, but does build more bulk; the levels of low-rpm pressure and "reps" of pedal strokes done in cycling for tens of minutes are definitely in the high-rep/low weight gym regimes. To really augment your strength, better off doing weight workouts with leg press, knee extension, knee contraction/curls, and squats. It definitely helps cycling performance.