Steve-O -- Oh I'm Sorry ... I Didn't See Any 'No Backflip' Signs On This Bridge

Steve-O insists police should be THANKING him for doing a high-flying backflip off a San Antonio bridge this weekend -- into four feet of water -- telling TMZ, thanks to him, the entire city now knows ... that bridge should really have "no diving" signs on it.

San Antonio police are currently investigating the incident -- a rep for the SAPD tells us, they're trying to determine how Steve-O should be punished ... because backflipping off that bridge isn't just illegal, it's extremely dangerous.

But Steve-O says that's BS -- telling us, it's dangerous because the city never posted signs on the bridge, warning people that the water below is only 4 feet deep.

He adds, "They've given no indication that people shouldn't jump, and done nothing to warn anyone about the serious dangers of that shallow water. Very irresponsible, indeed."