[remotemass (OP)]

Seawater erodes all sorts of materials including metals but would a pipe of diamond survive for a long-time in the ocean in permanent contact with the seawater?

- remotemass

[Bored chemist]

Probably pretty much forever.
Certainly long enough to get covered in barnacles.

[evan_au]

Metals degrade because they get oxidised by dissolved oxygen, and the water acts acts an electrolyte - and it happens much faster if there is an electrical connection to another metal. So ships and undersea pipelines have a "sacrificial anode" (eg zinc) that gets eroded instead the steel pipe. But if the sacrificial anode is not replaced before it is eroded, the steel will start to corrode.
That's assuming someone doesn't put a demolition charge on the pipeline, as appears to have happened recently in the Baltic Sea...
https://en.wikipedia.org/wiki/Galvanic_anode

Diamond is very strong due to its interlinked tetrahedral crystal structure. But at the surface are "loose" carbon bonds, which are much more reactive. Dissolved oxygen will react with these bonds, exposing yet more unpaired carbon bonds, and slowly eroding the diamond.
- These loose bonds will also react with hydrogen, producing a self-terminating chemical reaction (a hydrocarbon surface)
- But jewellers often clean diamonds with hydrofluoric acid, which will react with the loose bonds, producing a very non-reactive fluorocarbon surface.
- Another option is if the tetrahedral diamond carbon structure could coated with something like a graphene carbon surface, which has no loose bonds? But mechanically, it would be fairly weak compared to the underlying diamond structure...

[Bored chemist]

if the tetrahedral diamond carbon structure could coated with something like a graphene carbon surface,

The graphene would act as kindling.
https://en.wikipedia.org/wiki/Graphene#Chemical

graphene carbon surface, which has no loose bonds

Graphene is entirely covered with loose bonds and is rather reactive.

But at the surface are "loose" carbon bonds, which are much more reactive. Dissolved oxygen will react with these bonds, exposing yet more unpaired carbon bonds, and slowly eroding the diamond.

If that was significant, the same process would occur in the absence of water  and indeed, it does. Diamonds will burn in air.
But the reaction is so slow that you need to work on it.

Adding water will slow down the oxidation.

Diamonds in seawater are pretty much "forever".
The sun will boil the oceans before the diamonds dissolve (unless you make them impractically thin).