I work in the sector (offshore renewables and O&G), here are a couple of interesting ones:
1. Short installation window: they can only install this machine when the tides are changing (i.e. not much flow), so they only have a couple of hours to collect the moorings and it hook it up to the grid
2. Fatigue: loads of areas, not just the turbine blades... variable loading on the mooring lines, vortex induced vibration of the power offtake cable, wave induced motion on the offtake cable
On the two you mentioned:
Marine growth (barnacles etc.) is generally manageable with anti fouling paints, or PTFE (teflon) coatings, wipers and bearings around the things that you don't want to seize up. The animals will have a hard time getting a footing in this high flow environment anyway, it'll be pretty well self cleaning.
Saltwater / corrosion is also manageable with coating systems, cathodic protection and large corrosion allowances.
While these two do cause issues, they're kinda "solved", you can generally apply the same straightforward techniques and they don't take up as much engineering effort as solving the installation, system design and fatigue.
I’m guessing salt corrodes metals and barnacles grow on them which make the moving parts seize up.
What are some of the less obvious issues that they face?