  Deployed optical clocks will improve positioning for navigational autonomy,
provide remote time standards for geophysical monitoring and distributed
coherent sensing, allow time synchronization of remote quantum networks, and
provide operational redundancy for national time standards. While laboratory
optical clocks now reach timing inaccuracies below 1E-18, transportable
versions of these high-performing clocks have limited utility due to their
size, environmental sensitivity, and cost. Here we report the development of
optical clocks with the requisite combination of size, performance, and
environmental insensitivity for operation on mobile platforms. The 35 L clock
combines a molecular iodine spectrometer, fiber frequency comb, and control
electronics. Three of these clocks operated continuously aboard a naval ship in
the Pacific Ocean for 20 days while accruing timing errors below 300 ps per
day. The clocks have comparable performance to active hydrogen masers in
one-tenth the volume. Operating high-performance clocks at sea has been
historically challenging and continues to be critical for navigation. This
demonstration marks a significant technological advancement that heralds the
arrival of future optical timekeeping networks.
