Like a spitting image of The Situation, Justin Bieber once again blew his top ... and flexed his abs HARD for some shirtless photos on a boat in Connecticut.

Complete with soggy sagging basketball shorts and Marky Mark inspired underwear, Bieber hung topside with his posse  -- Including Little Za and Twist -- to soak in the sun.

Dude seems to be allergic to shirts these days -- he's been showing more skin than that guy from "Silence of the Lambs."

He's ripped though ... so good for him.