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We have a 5 kw Onan diesel-powered generator that sends out 120 volts.

When you said generator, you really meant it....(grin).... Your Onan in any sailboat environment I can imagine, is putting out a cleaner 120 volts source than your home, marina or office environment, as there are fewer devices starting and stopping operation to produce voltage sags or voltage spikes.

You will not need surge protection.The power supply in computer equipment produced in the last 10, maybe even 20 years, use what is known as a switching power supply. That means that the incoming voltage to the computer is chopped into very short duration pieces, and those pieces are turned into an amazingly clean DC voltage from even a very electrically noisy ac line voltage source.

That said, if you want to put an office quality surge protector in front of your power cord, go ahead, it is cheap peace of mind if you have lingering doubts.

I am curious as to why you want to run your Macs from the generator at all, unless things are set up on your ship ( I suspect your boat may be a lot closer to ship size than my present little boat) so that the generator is on all the time anyway for other reasons.

Have you considered using small inverters plugged into cigarette lighter outlets to power your Macs? Or even a larger inverter to handle all your AC needs while the generator is off?