Dips to the upper 20's won't harm anything. Not cold enough for long enough, especially since it's in the water (radiant heat from the ocean).
Now, if it gets colder/longer...
-- OB's are totally fine - just give them a few minutes to fully drain before tilting. You can leave them down, if you want - you'll just go through the anodes faster (no big deal). Ideally, antifoul the lower units if you're going to do that.
-- Other things (fresh water, water heater, AC, genny) could have problems. It might be best to winterize those things.
-- If you have it setup to have easy access, it would be OK to use the raw water washdown and just drain/winterize each time as it's pretty straight forward and doesn't take long... or at least you could set it up so it doesn't.
-- Same with livewell supply lines - all depends on your access and how you have them setup. Livewell drains are easy.
-- Most head systems aren't too bad to quickly winterize, either (not sure what type of system you have). As long as you have access to pump outs when needed. But some extra AF in the holding tank can help.
In the end, though, there is no magic number where it is/is not a problem. This is a "better safe than sorry" thing.
You might be best to access locals with similar setups - locals that have been around for quite a while - to see how they have faired over the years. But keep in mind if you get a freak cold snap, you run the risk of damage. You'd have to act quickly if you're doing it yourself... and you may not be able to hire someone last minute.