The islands would be pushed and lifted by the tides. Tidal generation would be worse on these islands than on a regular island. There might, however, be a quirky way to generate power.
A tide is essentially the moon pulling the water to it, which creates a current. Your free floating island would be affected by the same gravitational pull, and by the current created by the tides, so it would move in the same direction, but maybe less fast than the surrounding water.
Tides also cause changes in water level, as the water moves. Your island would just rise with these, however, as it keeps the same boyancy, so no observable increase in sea level would occur
So, here's why it'll be worse for tidal generation - your island is moving in the same direction as the water, so, relative to it, the water is moving more slowly. Slower water = less energy generated.
However, wave power would still be fine.
Waves are too small to move the island, so they'll break on the shore.
Beaches will be more difficult - a horseshoe shaped island might accumulate sediment in the middle.
The bit that I can't figure out, at least without someone willing to lend a wavepool and a nice island model, is if the islands tilt with the tides. I'm guessing they would, which would do an excellent job dislodging sand, and make beaches uniquely unlikely to form here. You have another source of sediment removal (west edge of the island goes slightly further underwater than usual, and as it rises, you'd see a whole load of water movement.
Thinking about this, this might be the source of a different generator - an island with a horseshoe bay would tip down as the tide hit the horseshoe edge, filling it more deeply with water than normal. As the tidal pressure equalizes, it'd tip back to upright, causing a large rush out of water out of the bay. It's not a tidal generator, per se, but relies on the rocking of the island, and the movement of water into the bay.