Mainly, SpaceX operates a drone ship that was used as a landing pad to recover boosters at sea.
For the time being, the at-sea landings have all been for SpaceX's Falcon 9 rocket, that carries a smaller payload.
The Falcon 9 rocket is compiled of three boosters, with one on each side of a centre core booster. On Monday, April 15, the two side boosters returned to land at Kennedy Space Center, while the center booster rendezvoused with the drone ship for recovery at sea.
All the boosters landed as expected. Yet, the centre booster experienced a mishap during the drone ship's return to port.
Therefore, SpaceX commented
Due to rough sea conditions, SpaceX’s recovery team was unable to secure the center core booster for its return trip to Port Canaveral. As conditions worsened with eight to ten foot swells, the booster began to shift and ultimately was unable to remain upright. While we had hoped to bring the booster back intact, the safety of our team always takes precedence.