There is a lot of water out there.
* Total ocean volume: 1.3e18 cubic meters, 1.8e21 oil barrels equivalent [1]
* Current oil consumption: 35,442,913,090 barrels per year [2]
* Oil share of total energy consumption: 34% [3]
* Number of years of reserves at current consumption levels: 17,577,561,935 years.
Assumptions:
* Consumption levels are flat, definitely false.
* Energy cost of retrieval: 0, definitely false.
* Retrieval rate: 100%, definitely false, as the concentration dilutes over time.
[1] https://ngdc.noaa.gov/mgg/global/etopo1_ocean_volumes.html