Welcome to Ribnet.
Firstly, Your Yam wiring colour guesses are correct (well, according to the info I got off the net a while back, and certainly agree with my old '79 vintage 55 Yam colour scheme).
I agree with your connections too. I have a Suzuki Tacho which has told me how fast both the old Yam and the current Merc were spinning, so there should be no reason why it shouldn't work.
A simple test might be to switch it to a different pole setting, & see what happens. It may be as simple as the 6 pole connection on the switch is dodgy or not fully clicked into place. click it back & forth a few times & see if it decides to work. If it works at a different pole setting, but tells porkies in the RPM dept. - that would at least prove the boat wiring is OK.
Most of these "olsdschool" tachos need 12V to run, and basically count the pulses from the alternator, which form memory should appear at >4.5V (AC). Might be worth checking the voltages at the tacho, and clean all the connector pins, as there may be some weird capacitance thing going on.
Only other option is to get a multimeter & check for supply & signal voltages (Supply DC, sigbnal measure it on "AC" setting - it should read >4.5V) and chase it right back to the engine if need be.
Like I say, there's no reason why it shouldn't work.