Actually, a bolt cutter can be very quick and efficient on chain. I would choose that ahead of trying to make two cuts with a power tool while trying to hold it firm enough with vice grips. However, if I were buying a tool primarily for this purpose, I’d get a small hand powered hydraulic rebar cutter. (Not the battery or electric ones.) That’s the cats meow. Short handles. Quick pump up. Less than $100 for a cheap one at place like Harbor Freight.
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And a couple quick tips for those who already have access to bolt cutters. It isn’t as hard as you would picture, because we have the advantage of the chain being at deck level. It’s easy to grab a link in the jaws, set one handle on the deck, and use your weight to push down on the other handle. If under a different scenario you can’t put one handle on the deck, put one handle against your upper thigh/hip area and use two hands to pull the other handle towards yourself. Both are fairly easy, and can be done squatting on a pitching deck. Seldom should you need to try to pull the two handles with both arms outstretched if you learn other techniques. Both sides of a link can be cut in about 20 seconds. (35 years in the fire service cutting chain and locks, and between that and side jobs, hundreds of cuts in rebar.)