The really hard part is the combination of the very short range of light transmission underwater and the very large size of the oceans. How many centuries are you willing to spend on the project?
The MH370 search used sonar, which works well underwater but has quite short range so it involved driving ships around on the surface dragging sonar arrays backwards and forwards for years.
I think this is one of the systems used during the MH370 search, apparently it can scan 192km² per day.
There are 161,000,000 km² of ocean, so you'd need at least a million days or 3,000 years to scan the whole thing, give or take.
That sounds extremely slow but presumably now that they've built one the cost to build 100 or 1000 would be achievable if someone (or some government) had the interest.
I would think they would need a version that doesn't need the ship nearby to scale - but would still guess this is something that will be accomplished on the order of 30 years from now.
I think this is one of the systems used during the MH370 search, apparently it can scan 192km² per day.
There are 161,000,000 km² of ocean, so you'd need at least a million days or 3,000 years to scan the whole thing, give or take.
http://www.slhydrospheric.com/prosas60_spec.html