The Falcon 9 still has to deal with heat cycles, vibration, and soot/chemicals byproducts from the engines. The heat cycles and vibration lead to work hardening and cracking or softening of components. Soot can and will get into everything and destroy bearings and connectors.

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.