Can the ocean be used as ground?

In principle, yes - all you need is a sufficiently good connection to ground, and saltwater will certainly provide that. Single Wire Earth Return power transmission systems can use this for return power; they usually use titanium grid electrodes to prevent corrosion.

Rocky soil does not offer a good resistivity, so that typical methods of driving electrodes into the soil will not reach a satisfactorily low earthing resistance.

It is assumed that the supply earthing is coming from the distribution network (at some extent utility), the purpose of local earthing is protection against lightning. It is a tall tower, on an island, so the risk assessment will come up with a large equivalent capture area, and correspondingly the need for an efficient Lightning Protection System. Required earthing resistance is 10 ohm or less (IEC 62305). If expecting very large lightning current (near or above cat. I of 62305; e.g. it happened in Malaysia ... >300 kA recorded!), resistance shall be correspondingly reduced.

Earthing with electrodes in the sea is possible and acceptable, provided that:

• the earthing resistance can be measured (inspection pits and suitable method ... remember that you do not have good soil for auxiliary measurement electrodes, so you will need to adapt a method like Fall-of-Potential method to the sea : floating auxiliary electrodes with a buoy?)
• calculation by design maybe done assuming the sea water with conveniently good, but conservative, electrical conductivity: typically, it is said some S/m
• in case of lightning a large current will discharge through the earthing system to the sea: are there phenomena of gasification, electrolysis and chemical decomposition, suddenly increasing the effective resistance, exactly when you need it? don't know, interesting
• corrosion of conductor surface: copper will be prone to heavy oxidation increasing the resistance; probably when discharging the overvoltage will perforate any oxide barrier, but during measurements results may be "false". Moreover, corrosion will bring to a reduction of cross section. Be careful also of ethero-junctions and combinations of different metals, that might corrode quickly the joints. Probably a steel cable directly connected to an earth bar in a suitable dry protected cabinet is the best solution.