I also noticed fluctuations when going through different temperature currents of water. When we would enter a warm current speed reduced slightly, then pick up in cold current.
It may have been current speed based on convective flow like diurnal wind changes, that I couldn’t sense, but I could sense the temp changes.
Currently boatless
2020 SA 450 Wife calls it White Cloud. Said it makes her feel "Classy"
2017 Sanger V215sx. We call it Viagra because it's the little blue pill that gets everyone up (Sold)