On the back of your existing key switch will be a yellow/red wire that fires the start solenoid when you roll the key to Start.
Pull power from a switched circuit (purple) into whatever push button you want to use. Splice the switch's output into the yellow/red circuit on the back of your key switch. That will fire the solenoid exactly the same way the key switch does.
Be sure to wire it in parallel or you'll need to use both to start !
The Cole Hersee pushbuttons (M490, M626) work well for that and you can get them with a boot.
There's not much current going through it so you can use just about anything as long as it's robust enough to be on a boat.
Your start-in-gear protection and lanyard switch won't see any difference.
A lot of OEMs (esp Sea Ray) mount the key switch out of the weather and use a start/stop button on the dash (or the control) instead on their SmartCraft boats.
As to the nut on the switch... sikaflex the threads and the top of the switch body.
If you have a thin (brass, aluminum, uncored glass) dash panel, the inner nut might be threaded too far down the barrel, allowing the outer (trim) nut to bottom out before it tightens against the dash.