The Gronk cruise will be ONE GRONK DOWN ... 'cause Rob's brother Dan tells TMZ Sports he has to bail on the party boat because his wife is pregnant and they can't risk getting the Zika virus.
The cruise (which leaves Friday from Miami) has been the talk of the family for months -- in fact, Dan says he and wife Brittany had timed out their pregnancy (she's not due until June) so they could both go along for the ride.
But with the Zika virus spreading to the Caribbean -- close to where the cruise will sail -- Dan and his wife have decided it's too big of a risk ... and had to tap out.
"My wife can't go because she's pregnant and I can't go because it can be transmitted [sexually]," Dan says.
FYI -- medical research suggests Zika can cause serious birth defects in unborn children.
"I'm really disappointed. I was pumped for the cruise."
As for the rest of the Gronks -- "My brothers are all down in Florida ... they've already been going nonstop!"