It's best if you don't turn the nut from above. That tends to spin out the bedding compound as you tighten the bolt. Instead, just take a visegrip pliers and lock it onto the head of the bolt on deck, then wedge yourself under the deck with a makita chucked with the appropriately sized socket (you can get a hex-to-square adaptor (1/4" or 3/8" drive) for your power driver at any hardware store).
After you do all four bolts on one stanchion, going above each time to move the visegrips, you'll be ready to pay $1000 per pair for three more vicegrips, so you might as well bite the bullet before you get out to the boat, and buy a few more vice grips before you get started. That way you can goop up all four bolts, stick them in the holes of one stanchion base, clip vice grips on all four heads, and then squeeze in below to tighten all four nuts at once.
My stanchions unfortunately have flathead machine screws countersunk in the stanchion bases. It makes a much neater installation, but there's no getting around needing a helper for that job.
Good luck.