I'd still recommend a paddlewheel-style knotmeter. You can go to Airmar and purchase such an instrument in a retractable model. The thru-hull will be included ... and features an internal valve to limit in-flooding of water when the knotmeter is removed. You can remove it while in the water to clear any possible clogging and can remove it prior to trailering to prevent damage. In its place you insert a flush ended plug.
You could purchase an Airmar ST800 (NMEA 2000 compatible Waterspeed/Temperature instrument) for around $80 and get a Lowrance LMF-400 (NMEA 2000 multifunction display gauge) for around $200. Or, any other NMEA 2000 compatible display.
Once you move to NMEA 2000 intrumentation ... you won't be getting yourself into a situation where you have to purchase yet another dedicated display for a new parameter. You simply purchase a new instrument and the new data will be displayed on whichever display you own.
I just purchased a Garmin GMI-10. It is a multifunction display of the same size as standard marine instrumentation. It can be configured to display all sorts of data (wind, water, GPS, navigation, engine, tank levels, etc). Right now I have it displaying a graphical compass in the background with bearing on top. Below that is water speed and depth.
In summary, I'd recommend looking at NMEA 2000 multifunction displays and seperate instruments before you start buying dedicated displays/instruments for each parameter you'd like to see. In the end it will save you money.
1981 C&C 32