That is misfiring. With an outboard you will need a systematic approach. The first checks would be to drain the carbs and check to see if there is water in them. Then I'd check the spark plugs. If you can get a good pic of the spark plug electrodes and post it on here so that were looking at the insulators that would be helpful. If you're unsure what any of that means your next call should be to an outboard serviceman.

Anyone who is not set upto do so in fully equipped service workshop played with the timing?

When was the last technician/ work shop service?
When was the last time things like spark plugs changed/ gaps checked?

What happens when under load on the boat at sea?
Does it just miss once rpms get up?
Once speed rpms up does it miss and run out of power?

Check air valve in fuel tank open
 Check all fuel line joins and line for any cracks.
Check filters, including the under the cowel