So Garmin have very lazily responded by copy and pasting the web material into an email . But it does back up what @Maverick was saying.
Hi Neil,
Thank you for contacting Garmin Europe.
In certain environments, namely salt water, Garmin recommends painting the transducer with a water based, anti-fouling paint.
Anti-fouling paint helps to hinder marine growth and maintain transducer performance. This also helps in boat maintenance by lowering the costs associated with hauling a boat for cleaning.
Garmin recommends only using a water based, anti-fouling paint. Ketone-based paints should never be used. Ketone-based paints can weaken many plastics and possibly damage the transducer.
Please note: Garmin does not recommend painting the paddlewheel portion or any moving parts of a transducer as this can cause restriction.
If there is anything else I can help you with then please let me know
Kind regards,
Robbe
Marine Technical Support
Garmin Europe