Tom Brady ain't the only quarterback staying in great shape in retirement -- 'cause Eli Manning is lookin' like he's ready to make a comeback ... showing off his solid physique while on vacation with his friends!!
The 42-year-old former signal-caller was aboard a yacht in Portofino, Italy this week ... and he was joined by his old center, Shaun O'Hara, Capital Partners CEO Ben Weprin, and their spouses and friends.
Manning unleashed his inner child as he played with pool noodles, sea scooters, and, of course, a football .