That's not how buoyancy works. You don't need water "underneath", you just need to submerge the object in water to any degree, fully or partially, whether it's floating, or half way down the water column, or touching bottom. It makes absolutely no difference at all to the buoyant force, which will always be equal to the weight of the volume of water displaced.

To understand why, think of it on a reduced scale. Instead of imagining an ocean, say you have a beaker of water on your desk. The beaker is half full. Make a mark on the beaker corresponding to the water level.

Now take some long heavy object like a steel pin and start lowering it into the water. The water level will rise, because the steel pin has some volume and is displacing water. Water is incompressible and has nowhere to go except up, so the level will rise. The more you submerge it, the more the level rises, as illustrated in this picture.

It takes work to lift up the level of water against gravity. So what's happening is that the steel pin is losing weight (not mass) equal to the weight of water being forced up against gravity. It doesn't matter where the steel pin rests in the water column, whether it's partially submerged or fully submerged, whether it's at the surface or half way down, or hitting the bottom. So long as the water is displaced upwards, that steel pin weighs less by an equal amount as the weight of water displaced.

It's very easy to verify this if you have a pin and beaker as shown in the figure I linked. All you need is a spring balance to suspend the steel pin from, and you can observe the steel pin losing weight as you submerge it further. It doesn't magically regain the lost weight when it hits bottom, it is still lighter by the amount of water displaced.

If you don't have a spring balance, use a stretchy rubber band instead. Measure it at rest, then measure how long it is when the steel pin is suspended in air. Then measure its length again when the steel pin is partially submerged and when it hits bottom. You'll see that it shortens by an amount corresponding to the water displaced.