Hi there I live in Australia.
My. 2008 115 optimax is stopping dead at 5000 plus rpm.
It sounds a long beep as it comes to a halt, no reduced revs just a dead stop.
I had checked all the common things; I’m a Marine Engineer by trade. Then realized it was still under warranty and with out the diagnostic laptop and software I was limited to what I could do. Initially it was stopping dead in the water at random rev ranges. When the Merc mechanic plugged in he found 50 plus throttle position sensor faults and a couple of engine too cold faults. They replaced the throttle position sensor and fitted new thermostat seat as advised by Merc Australia. When I next took it out it run o.k but I didn’t go far or fast. The next time I took it out I motored around for an hour or so at lower revs, then when steaming home I was at top rev range for about 20 – 30 minutes and the motor stopped dead no warning and had one long beep on the alarm ( I don’t have smart craft) The worst thing being it sent my 4 year old boy flying into the cabin. Now I can’t trust the boat as it is dangerous. The motor restarted and died then restarted run a little more then died, restarted and run home 4000 rpm no problem. Took strait into dealer thinking that a fault must be logged on the ECU but they informed me they couldn’t find any thing, they also sea tested it with no fault, I am starting to lose faith as I was sure a fault code should of been recorded. Anyone else have any ideas?
Thanks in advance Clayton.

With an alarm it should have set a code.
try 2 things:
1) run motor on a remote fuel tank
2) rum motor on a separate keyswitch harness.
Effectively eliminating the boat from the equation.
If the problem still exists the next thing to do is run the motor with both the fuel/air pressure gauges and either the Mercury DDT or CDS installed.