GW, you are right in that paddle wheel speed transducers will require very regular removal for cleaning.
Debris and marine growth are both issues.
Easy enough to do, especially in a smaller open boat. Make sure you have the blanking bung handy tho, as a surprising amount of water comes in! However, mounting this thru hull perhaps not so easy in a BR?
My new to me Raymarine uses Transmar brand transducers (model about 5 years old?) as do many others. These have a hull hole with a small non return flap valve to limit the inrush. However, due to past experiences (fouling/debris unsurprisingly also affects the valve!) I don't trust this (or indeed, my crew to assist me..some get quite spooked), and have just ordered a blanking plug. £40! You then do a speedy swap over, and can clean the paddle wheel at your leisure, undistracted by an open 30mm hole in the bottom of your boat!
This unit has a dual function speed thru water paddle wheel, and water temperature (of no interest to me, a fish finder function?).
It has a separate depth head, in another thru hull mounting tube.
I'm investigating using this hole for a NASA electromagnetic log...
NASA Clipper do an electromagnetic speed log, with no moving parts (£350ish?) However, note that they still say that this needs cleaning.
Others do ultrasonics etc, to eliminate the pesky paddle wheel which WILL CLOG UP.
Speed thru water is, imo, important especially for racing...I seem to recall that many classes didn't allow GPS years ago (on a race boat, we "invested" in an early TackTick network, which i think was appx £2200, about 20 years ago! That was paddle wheel only. I seem to recall top flight boats had very fancy ultrasonics?) so STW was vital. STW also reacts faster than GPS speed (SOG) so is better for trimming to max boat speed I think?
Of course, both my navtab and phone have GPS "speedo" displays, so I can retire my trusty Garmin GPS72...if only I was smart enough to figure out how to transfer all my historical waypoints/routes...