My research and (limited) experience is leading me to believe that quality marine electronics last quite a while (10-15 years) if cared for properly.
You may get frustrated with the rapid pace of technology that quickly renders your equipment obsolete, but it will still function for many years.
In the case of anamometers, depth finders and knot meters, I think the instrument head tends to fail before the transducer. Some of these companies, like Signet and Datamarine still offer repair services on even very old, obsolete products.
My advice is to either remove the instrument heads and store them when not in use, or at least have covers for them to protect them from blistering heat and cold winters.
This is all anecdotal though, I'm sorry I don't have any hard figures to back it up.
S/V Old Shoes
1973 Pearson 30 #255