If you're referring to the speedo port of an outboard lower unit...yeah they suck, and a tiny leaf fragment will put you out of business. An actual pitot though has an oblong port and will still read accurate when partially blocked, since regardless of the size of the port pressure remains the same. Unless you smack something big enough to kick it up or break it then it always reads, and if it reads at all....then it reads accurate and is instantaneous when speed changes.
Of course none of this matters at all 🙄 I just like to point out that old school simple technology is often the most reliable. It will definitely read 1mph, and .01 mph just flat out doesn't matter unless you are a powerboat racer trying to set a record.