Even the spray from the ocean combined with the heat from the latent heat in the craft can lead to accelerated corrosion.
They're shooting for a final touchdown speed of 2m/s which seems fairly slow but that's still a lot of mass to stop very quickly. The ocean helps soften touchdown for sure.
It would not surprise me if the turn around procedure was to r&r the entire bottom third of the craft and (possibly) the fuel tanks. The removed components could get inspected and rebuilt and flown again.
It won't be anywhere near the scale of the shuttle turn around but it won't be as simple as hosing it off and fueling it up.
SpaceX has of course considered all of these things and it will be an amazing thing for space travel if they pull it off.
The ultimate goal is to land at least most of the cores on land, not the barge, in part to avoid this.
They basically want this to be a smart rocket. Something that can just launch payloads into and out of orbit, and come home like a good dog after a long day of work.
I'm confident SpaceX themselves would have tested multiple engines to failure with many, many re-light, shutdown, re-light cycles so they have a good understanding of what's going to happen when they try to re-use one of these things.
There is no way they'd be going to all this trouble to design a rocket that can come back, all the trouble of giving it someplace to land if they were not already confident the engine can be used at least a few times.