I went through this when we bought our 310, there was no plug for the knotmeter transducer. I called the P.O. and he had just used a tapered rubber plug in the hole when cleaning it.
When the transducer failed and I ordered a replacement, it turned out that Raymarine didn't sell the plug as a separate item, it came with the complete instrument kit, but not with the transducer alone.
They were able to give me the part number for the plug, and I found it through a Raymarine distributor. That was 4 or 5 years back, and the paperwork is on the boat, not here, so I don't have the P/N handy.
Depending on weather/work schedules/life in general, sometimes the boat would sit for a month or two, and the wheel on the transducer would be fouled. Now I leave the plug in until I'm going sailing, then swap it out and sail with a clean paddle wheel. Consider that if the boat sits much.