An aqueous feed stream having a first total dissolved solids (TDS) level is flowed to a forward osmosis separator. The aqueous feed stream includes seawater. An aqueous draw stream having a second TDS level is flowed to the forward osmosis separator. The second TDS level is greater than the first TDS level. A disposal stream and an injection fluid stream is produced by the forward osmosis separator by allowing water to pass from the aqueous feed stream to the aqueous draw stream through a membrane of the forward osmosis separator based on a difference between the first TDS level and the seconds TDS level. The injection fluid stream is flowed from the osmosis separator to a subterranean formation.