They would be easier to service than off-shore wind turbines. You just need a light cable attached to a buoy, the service ship goes to the boy, pulls up a heavier cable with the light cable, then pull up the whole structure.
Come to think of it, the turbine pods may include a ballast tank that can be evacuated, to adjust the height in normal operation or to pull itself up to the surface for maintenance.
I'm not so sure. For a wind turbine a lot of maintenance can be done without any additional vessel / cranes. If you need to do a major component change you do it with a similar vessel to the ones which installed the turbine. For a subsea tidal turbine you'd need some frame or similar to land the tidal turbine onto, you'd need to clean it before anyone could access it for the maintenance etc. Generally speaking if you have something subsea it should be designed to be maintenance free as recovery costs are so high (like subsea oil and gas production systems).
Come to think of it, the turbine pods may include a ballast tank that can be evacuated, to adjust the height in normal operation or to pull itself up to the surface for maintenance.