When I took my boat on, the erratic operation of the rev counter while under way was a source of considerable annoyance, Sometime working correctly, most of the time reading at around half true, other times not reading at all.
Quite separately, when asked to check the state of my batteries, the marina diagnosed a faulty alternator (but i don't recall quite why) and replaced six months ago. Since then the rev counter has been absolutely fine apart from the occasion a couple of weeks ago when it started under reading again miles from anywhere. I thought here we go again, but three minutes later the belt shredded. I did wonder why it had gone so quickly, but fortunately I had asked them to put a spare on board when they replaced the alternator.