Somewhere around the middle of the fight
That said, the first section can take a while, as there is a lot of mob-clearing Lost Ark Gold, and players don't have their optimized Skills down yet most of the time. Plus, for some reason, the Purple Team and Green Team checkpoint loading after the first batch of mob-clearing always seem to lag and cause everyone to run around in circles wondering if the Dungeon is actually bugged this time or if it'll fix itself in a minute.
Deep Sea Orboros, for all intents and purposes, is just an underwater Hydra or fishy Cerberus. It's the protector of this "sacred place" that's put there to stop anyone from getting in. The actual fight itself is so incredibly basic, mostly because Smilegate knows that players aren't using the Skills they've spent hundreds of hours gaining muscle memory with. That said, there is still one thing to keep in mind, and it's the Shark Summon.
Somewhere around the middle of the fight, Orboros summons this bipedal Shark Monster into the arena who has an icon on the minimap. Once this creature spawns, everyone should immediately switch to killing it instead, ignoring Orboros for the most part. The Shark has a variety of its own attacks, mostly tail swipes and biting attacks, but it's really just there to punish players who don't take notice of it. If it's left alone, the boss isn't buffed or anything, but if it's killed, the party gets a massive damage buff that will melt Orboros' remaining Health Bars.
All that said, it's still very possible to die during this Boss, as Orboros has a good variety of wide-reaching attacks and stage hazards.
Bright blue Lightning Orbs are summoned around the arena from time to time and cheapest Lost Ark Gold actually show up throughout the three Dungeons of the Gate of Paradise. These things explode and leave a damaging field behind if they aren't broken beforehand.