I checked out Takctick - nice. Except:
(1) I would beach the boat - may damage underhull sensors.
(2) Price is also an issue.
The whole boat is hand built by me, 6m length, 0.5m width (Tamanu sailing outrigger). So going old school for some items would be beneficial.
I guess throwing a log and measuring knots would also be a way
There are transom mounts where the transducer can be pushed up on a sliding bracket.
The other alternative is a small sub keel to protect the transducer. The paddle wheel only sticks out a few mm (maybe 1/4 inch) so it does not need much
I think Tacktic are a bit overpriced and a small sealed battery with a cheap log or combined log/ depth transducer would be a good option
My choice would still be a GPS giving STW and a spare unit with fresh.
Unless you are racing, STW is not really necessary. SOG is more useful. Many large boats have such poor calibration on the log that they use the GPS for their primary speed information and pay little attention to log.
If you set up a good quality log. position it well, calibrate it and keep it clean you will get good information on tidal effects, but many cruising yachts give up.
On a much simpler vessel, such as yours, I would not hesitate to forgo the difficulty and rely on the GPS for my only speed information.
Pulling in a trailing log, using the dividers, handbearing compass and map to plot your position sounds romantic, but its hard work and difficult to do in a small boat. You will use the GPS and if it fails you will be glad you have a spare.