There are some coatings (MDR used to make one). I haven't tried to use that in decades. I leave ours "bare".

I just pull mine unless I'm going to be sailing in a couple of days, or so. I think that is a much better practice because even a little build-up of growth that you start to get over the weeks in the water affects the calibration of the knot meter.

EVEN high end transducers, clean a whistle are only approximate! They can be calibrated accurate within a range -- but variations in the tack, trim of sails, general velocity, etc. Ours is on the centerline of the boat about 5' in front of the mast. I can get it zoned-in at, say 8 knots. If we're poking along at 4 knots, it's off by 4 or 5%. When we start going approaching 10 knots, it understates the speed a bunch. It's different in a seaway depending on the wave conditions. It' a good indication of relative speed through the water, but not absolute speed through the water. (i.e. trending)

B&G tried, over the years, to sell the ultrasonic speed measurements devices using the Doppler effect reflected off the keel from a sensor forward. I really didn't do the job either.

For motoring in relatively flat conditions, the transducer can be set more closely.

The Walker Log (towed, very expensive and vulnerable shark bait) really is somewhat better because it's in "free[-r] water" that isn't affected as much by localized flow across the hull in front of the transducer.

This of course may not be your issue if your speed is within a narrower band.