As shammydodger says, a cucumber is 96% water.
The total dissolved solids in seawater amount to about 3.5%, the greatest constituent being salt - ocean water contains about 31% sodium, which is equivalent to about 79% sodium chloride (salt). It's as well to remember that salinity varies throughout the world's oceans at any one time due to ocean currents, evaporation, inflow of fresh water, snow etc.
In addition, some areas of the ocean are low in salt such as the western pacific seaboard of the USA and the Polar Regions.
Seawater also contains varying amounts of undissolved insoluble particles, organic material, microorganisms etc. which again varies from place to place.
When all these factors are taken into consideration, it's likely that a cucumber has a marginally higher percentage of water in a given mass than seawater, although as shammydodger says, it could be a close run thing.