According to https://pubs.acs.org/doi/10.1021/es1012752 the seabed area is roughly equal to the surface area of all oceans. They have a table with 361,383,969 km2 so let's use that. Assume you could cover 0.001% of the sea floor with turbines: 0.001% is that which is accessible to land and not too deep and not too cold and not too stormy, but still mostly beyond what we can do technically, but that is ok, we are estimating a "resource" for the future. However, you still need to guess at the occupancy of that overall area because you need to place the rotors so they don't hit each other. They need to be at least 20 m apart in each direction, but you probably need space for services and maybe to stagger them so they don't "shadow" each other. So let's assume each one occupies a surface area of 40 m x 40 m. What is the total power from all these stream generators? That is your "resource".