On the back of your tachometers, there is a selector switch for the number of cylinders - it is black, if I recall correctly. Rotate it back and forth several times, then return it to it's previous setting. Doing so cleans the contacts, and often restores a tach to proper operation. If it reads high, but your sync gauge still works, this is very likely the culprit.
The horn should be easy - remove the housing, and verify that the horn is getting power when the switch is depressed. If yes, the horn is dead. If not, it is very likely dirty contacts in the Carling switch on the dashboard. I have had these apart many times, and we can check the switch contacts at LL, if you like.
The slow leak in the engine room - I need more info. Could be a shaft seal starting to let go - Tides Marine recommends replacement approximately every 6 years, so if yours was done back in 2006 or so, you are due. If it is engine related, can you provide a bit more information? Could be a heat exchanger, or other raw water related component.
I can't help with the haler - your memory is beyond my control!!!
Dale