Suzuki 2.5 rough idle - Attempting to fix by eliminating ignition problems.

We start at Aberdovey hoping to launch but the winds just become dangerous. 2nd plan is to fix the motor in the garage. I clean the old spark plug and re-gap - I then try a new spark plug in the garage off the boat to fix the idle problem. The idea is i'm trying to eliminate ignition problems for the rough and non existent idle. Failing that my last ditch attempt is to launch the motor on the boat and give it a hot extended run. With all the rain we've been having I soon learn how the most calm and sheltered waters can catch you out. I get caught in flood water with a defective motor avoiding fallen debris!