Re: Opinions on spark plugs / spark plug wires
Any good quality wire set will work with todays ignition systems. OEM would have the correct wire sizes due to the routing around the manifolds that auto applications don't have.
I would stick with using mercruiser oem plug wire sets, The fit is so much more covenant to install in all aspects + the fit of the wire boots at both ends are so much more secure reducing possible spark leakage especially at the coil tower.
The coil wire sometimes seems to be a prone issue to leak out of the coil tower if the boot is not a tight oem fit, at least so far in my experience that is.
Once you find the oem part number, you can usually find a really good deal on places like ebay, etc using the part number.
Also consider on how long your original one's lasted.
A good spark plug would be either AC delco or NGK. We would need your mercruiser engine serial number to comment on what plug number and gap you need.
Good luck.