I don't have one nor know about 2015 engines of that hp. However, in Merc. engines of earlier years and higher HP, if you have a voltage regulated output to charge your battery, and you should as you said it was a 25e...e for electric start, you should have a little 1 1/2" (roughly) square epoxy filled aluminum gizmo mounted somewhere on the engine block or attached plate, with the following wires emanating from it:
2 yellow (AC input from stator under the flywheel), 1 black (grounded to the engine block), 1 or 2 red (dc output to battery + terminal via usually the input high current stud on the starting solenoid, and "last but not least" a grey wire. This is your tach drive.
The tach will need 12v, ground, and this grey wire. If you have a Quicksilver Commander 2000 or 3000 up near control in the control's wiring harness you will find a brown and a grey loose wire with bayonet connectors. The grey one is where you connect your tach. The brown one is for an overheat light or remote indicator for over temp/low oil sensor fails.
For your power, you need to look around the control box for a purple or red with a purple striped wire to hook into. This is your switched 12v and is absent when the ignition switch on the control is in the OFF position thus preventing your draining your battery over time when the engine is not running. Ground will be any black wire in the control's wiring harness.
Your tach should come with directions on how to install and setup for your engine. Most larger Mercs use the 6P position if you have numbers like that on the rear where there is a screwdriver selector slot. Don't know if your 25 uses the same number of poles in the alternator or not. One way of checking your numbers is your idle speed should be around 675 rpm. If you get that number you ought to be good to go. Wide Open Throttle (WOT) then would then be in the 4500 to 6000 rpm range if your engine is running like it should with your current prop and boat/load.
