Gee, My neighbor's Boston Whaler Outrage has the ign keys inside the center console, and push to start and stop buttons on the dashboard. it makes the dashboard look "cleaner" with all matching switches.
Ign keys on boats can prevent theft, if the boat is in a public place, in the daytime and you are not gone very long. Other than that scenario, I cannot see them stopping any real theft attempt.
Larger boats (I/O, IB) usually have access to the rear of the dashboard, and a jumper cable and a screwdriver on the terminal's of the ign switch, can hot wire them.
Outboards with the key in the control require you to go under the OB cowl and disconnect or cut the killer wire and jumper the starter solenoid. Heck, you can simply untie the boat and tow it away.
If the push-button start pleases you, go for it. You should also have a SPDT switch to provide power to the ign wires for any gauges and for the electric choke. I wire ground to the switch as well to kill the ign when the switch is turned off.