Here's the scoop on the poop ... Saturday at SeaWorld San Antonio, one of the orcas, AKA killer whales, was diving when it very noticeably dropped a deuce near the surface and -- this is important -- RIGHT on the edge of the tank.

It might not have been a big deal, except the massive whale swam back around to flip it's tail and splash a wave of brown water onto the crowd down in front.

Normally, SeaWorld's faithful love getting soaked by the orcas and dolphins as they perform tricks ... but, this was a painfully different situation.

As one guest in the splash zone put it, “We expected to get wet sitting so close, but not like that. It smelled awful, and people were gagging.”

Not shockingly, video of the filthy encounter has gone viral, and SeaWorld is in cleanup mode -- saying the visitors who got crapped on were taken to a decontamination area where they got soap, bottled water and towels. As for clean clothes ... they could buy some in the gift store. Generous!

Officially, SeaWorld says, "We regret this unfortunate event and are taking every step to ensure the safety and comfort of our guests. Our team followed established protocols to minimize health risks and will review our procedures to prevent this from happening again."