Do I need to go to a mechanic?

[Gerald]

Another possible cause is that a spark plug wire is loose.....ie. not tightly connected to the spark plug.

To check....grab the spark plug wire [not while the engine is running] and see if you can easly turn the rubber part that covers the end of the spark plug. If it feels tight......then try pushing the rubber end onto the spark plug.

After checking the spark plug wires......like others said, go run the outboard engine in water under load to see if it backfires. If it still backfires [or even if it doesen't backfire] pull and clean or replace the spark plugs.

Depending on how much you use the boat..... spark plugs should be changed ever year or two.