Removing and replacing the transducer is rather intimidating at first but once you have done it a couple of times you will be fine. I wrap a large cloth around the base and it catches most of the spillage.
My thoughts:
1. The paddle wheel is extremely useful as it provides speed through the water. When you compare this with speed over ground you can see how much tide is with or against you. It also may be used to determine true wind as Paul Hutton points out.
2. My paddle is Airmar which was commonly supplied with Raymarine instruments. They get weeded up regularly. Airmar recommends antifouling the paddle wheel and all round the inside of the housing (see Anti-fouling Coating in the attachment).
3. When you refit the transducer, there is a procedure to ensure that it is correctly seated which is critical to avoid leakage. I have attached the instruction sheet rather than try to explain here (see Servicing the Valve Assembly Section 8 in the attachment).
Hope this helps.