Harry, first take into consideration what
charts you will be using. That may influence your decision. Though I'm not sure if all programs will accept all
charts or not. Just something to consider.
I use Polar View with the free NOAA charts for the US on my Mac. I'm very pleased with this low cost solution for my needs.
Polar View has an excellent color palette with the vector charts. It's not demanding on CPU or graphics resources and runs quite fast.
My second choice would be MacEnc.
OpenCPN is immature at this point on the Mac, but getting better.
Also, give very careful consideration to how you will protect your
laptop from
water intrusion. Especially if you wish to have the nav
software up and running while underway. It's turned out to be harder than anticipated for me.
At this point it's clear I would have been wiser to go ahead and add a new
chartplotter at the
helm to replace an out of date monochrome
Garmin. Damn hindsight.