Michael Arkinson -- the Sag Harbor police officer who pulled over Justin and took him to jail -- spent his Sunday Funday on a boat, about an hour north of where he busted JT. If you didn't know, he and his partner, Otis, are cited in the police report as the arresting officers ... but Mike's getting credit as the guy who brought Justin in for booking.

Anyway ... check out these photos/videos, obtained by TMZ, showing a sometimes shirtless Arkinson sitting in a power boat docked at a restaurant in Greenport, NY over the weekend.

Folks who were there tell us Arkinson was eating and drinking at Claudio's Waterfront ... and kept a relatively low profile in the wake of his big bust, although it doesn't looks like he was trying to hide here. Dude's just out living life ... undeterred by all the attention he got in the wake of last week's arrest of Justin.

Arkinson, we're told, was hanging with a handful of buddies -- and they spent a few hours at the seafood spot before heading back to their boat.

The guy's already well-known around Sag Harbor. Locals told media outlets he's a bit of a traffic stickler, and now he's even more in the spotlight -- a local celeb of sorts, if you will.

Based on what we're seeing here though, Arkinson's not too bashful ... and it looks like he knows how to have a good time on Long Island, even after his name's been blown up in the media this past week.

Don't expect to see Arkinson's body cam of his interaction with Justin anytime soon, though -- as we first reported, Sag Harbor PD is not releasing the footage ... at least not until their investigation wraps.

In the meantime, Justin's continuing his world tour ... and Arkinson's taking full advantage of having friends with a boat.