My engine has a pitot speedo pickup and it is totally useless! It reads nothing at all when you first set off, if you go up to about 30 knots it gradually catches up after a few minutes and then reads more or less accurate, then when you slow down it takes ages to drop back to zero again.
It is a complete waste of space, so I use the page on the GPS (Garmin GPS72) with a big speed readout at the top, on the GPS72 this fills the top 1/3 of the display and is easily readable even when bouncing about a bit.
I suppose the only disadvantage of the GPS72 is it is easy to nick, as it just sits in a plastic mount and all you have to do is undo a thumbscrew, flip a locking thing and off it comes (or just rip it right off!) but it is also easy to remove when you leave the boat.
I've calibrated my other GPS (an eTrex) against a measured mile and the error is less than 0.5% so you can't ask for much better than that
Get a clock, or a barometer, or something that actually works, to fill the hole