The former tight end was commentating on "Fox NFL Sunday" as he always does ... but, he decided to change it up a little in honor of Veteran's Day tomorrow -- throwing on a wet suit and flippers and throwing himself out of a helicopter.

Watch the video ... a real Navy enlistee jumps out of the plane first to show Rob how it's done -- before he leaps and lands in the water. It's only about 20-25 feet in the air, but, as one of the guys on the broadcast points out, that's higher than most people want to jump from.

Gronk wore a GoPro so the audience at home could get a good look at his face, and he made his way to the aircraft carrier where his colleagues were ... climbing up on deck and doling out high-fives to the assembled military members.

Of course, no Rob Gronkowski achievement would be complete without spiking a football ... and, Rob slams one down with authority.

Rob tells his colleagues he's feeling fine after the jump ... though he's a bit cold -- and, he shows love for the active servicemen and women who are constantly jumping out of planes and performing other unbelievable feats.

Worth noting ... Gronk clearly doesn't have an issue with heights -- 'cause last year, they convinced him to jump out of a plane to highlight veterans.