The pressures for such a creature wouldn't be that different from those of a blue whale, and most likely neither would its habits. However some things don't seem to add up, despite being observed in nature, especially the senescence part, which, while observed in nature, is more characteristic to cold blooded and smaller creatures, being turtles the only vertebrates with such a trait, with the only exception being the naked mole rat , which has its negligible senescence mostly attributed to it's lower metabolic rate and relatively ectothermic nature (they have trouble staying warm). However, for it to be giant, it'll need to be endotherm, as that's the case with most instances of such large marine creatures (true, colossal squids are an exception, but that's mostly due to deep-sea gigantism, and is more characteristic to invertebrates). So while large animals like whales do have less chances of developing cancer and can have long lifespans, this doesn't equate to a lack of aging altogether.
The second problem would be it's scales, or rather, it's molting. I don't think your creature would molt like snakes do, especially at such large sizes. Rather, I believe it'd be something more similar to a pangolin than a snake, with a body covered in scales that are naturally shed and replaced over time, because something like this giant snake would never be able to molt properly, and as a consequence would likely become mostly blind and die due to infections and other problems caused by said failed molting.
Lastly the bombardier beetle spray. I truly don't think that'd be an effective defense mechanism simple because:
1-it's underwater, meaning it'll spread and affect it as well
2- the sheer volume of chemicals needed for a creature that size doesn't seem worth it.
Other than that, nothing too otherworldly to exist, just don't make it a scaled up sea snake, understand that it will age and die and (preferably) look for alretnative defense mechanisms more in line with an aquatic environment (maybe something like the hagfish's slime).