Well... You could use sonar
On a slightly more serious note, radio wave control is an option. You could also tether to a buoy, and have that use Wi-fi or whatever. 1km sounds ambitious; what are you planning on doing? Maybe I could help you better if you explained your project more.
_________________FTC Team 6424, the 'Oly Cow
- Chief programmer.
FRC Team 4450, Olympia Robotics Federation (ORF).
and also quadrotors. Quadrotors!