The constraints of building classical hydropower projects in seas and oceans are locational and economical dependent rather technological limits. The following question provides some valuable answers. For example, the answer of Dr. Tony clarifies critical constraints in Australia.

https://www.researchgate.net/post/Why_not_building_hydroelectric_plants_in_ocean_water_ie_salt_water_Is_this_because_there_is_no_reliable_design_proposed_yet

The constraints of building classical hydropower projects in seas and oceans are locational and economical dependent rather technological limits. The following question provides some valuable answers. For example, the answer of Dr. Tony clarifies critical constraints in Australia.

https://www.researchgate.net/post/Why_not_building_hydroelectric_plants_in_ocean_water_ie_salt_water_Is_this_because_there_is_no_reliable_design_proposed_yet

https://www.renewableenergyworld.com/ocean-energy/tech.html

interesting

The world will need immense amounts of energy storage to transition to 100 % renewables. Sea Water Pumped Hydropower (SWPH) could be an opportunity.

http://euanmearns.com/the-seawater-pumped-hydro-potential-of-the-world/

Dear Moaz Allane , you may find a lot of good contributions and resources in this related thread of mine.

https://www.researchgate.net/post/What_are_your_thoughts_on_renewable_ocean_energy_such_as_wave_wind_and_tidal_energy

Thank You all.

After taking these major concerns into consideration.

Do you think that a new hydropower system design with relatively similar concepts of that in hydropower dams will make a difference? Mohamed EL-Shimy , Mushtaq Ahmad , Javier Menendez , Ljubomir Jacić , Srete Nikolovski

I'll comment inline to all your points.

There is no reliable design proposed yet?

There are some designs - similar to the axial flow turbines used in fast flowing rivers or spillways in dams.

II. Or because the exact idea has not yet been taken into account?

See above

III. Reservoir: impossible to build an ocean-based reservoir?

If you have topography next to the ocean, this is possible. One need to take care not to cause salt pollution near sensitive ecosystems.

IV. Environmental, Social, and Economical concerns: A possibility of Non-environmental emissions, Harm to sea life?

Touched on above. Mussels like to attach themselves in seawater pipes, so they are killed when the pipes are cleared with pumping shot through them. Some small fish and crustatians will get pumped through the system. Often, this will protect the eggs and more will hatch in the reservoir, than would have survived in the wild. This may actually cause a bloom in certain sea life.

V. Civil & Mechanical engineering: Construction of the plant in the middle of the Ocean/Sea?

An axial turbine is simply moored to submerged concrete blocks, dropped in the ocean and the turbine attached to those with mooring cables.

VI. Survivability of the building?

Submerged concrete blocks are quite resilient in sea water. Recently the cement the Romans made for ports, using sea water as one of the components, has been reverse engineered. This material offers superior tolerance to seawater.

VII. Hydroelectric equipment: Generators, turbines, pumps etc…. Installation and efficiency during operation?

Synchronous Generators have typically 97%+ mechanical to electrical conversion efficiency. The conversion from the water flow is not as efficient, but you dont want to remove all the energy in ocean currents, as some has to stay to bring rain to dry areas.

VIII. Salty water: Impact of salty water on Turbines, Pipes, etc….?

This process is known. One typically use 316 or better Stainless Steel and galvanically protect the structures/machines with zinc based sacrificial annodes.

IX. Grid connection & Electricity transmission: Transporting electricity to the mainland station?

Use submersible cable, which is moored to some mooring blocks along the way,

X. Operation & Maintenance?

Operation is simple, as one has a few days warning of what ocean current is going to be, so one can better plan. Maintenance works best when the system has some redundancy built-in so one can take one module out of service at a time to send your divers down there,

XI. Retrofitting process: Costs and its viability?

Retrofitting works best with commercially off the shelf equipment, Avoid the custom made machines, This will make it even more viable.

XII. Licensing & Relicensing Process?

Licencing is determined by the authorities of the specific province, state or country. One has to get the authorities involved early in the project to make sure they understand precisely what the project is about.

Unendurable of final costs: Unaffordable Project?

There are many financing companies out there that are drooling over investing into a green energy project, - just go shopping.

Ocean indeed has a lot of hidden power and latent resources.

The design phase of a marine hydrokinetic device (MHK) on Autodesk Inventor solved all the stress test of the design but the biggest hurdle and lack of any real solution to deploying these devices off the Cape is the amount and speed of biofouling on the unit rendering it ineffective. No project can withstand the invasive species that will quickly accumulate and weigh it defenseless.