Metal Alloys and Chemical Hydrides seem to be up there in 6 kWh/l zone though https://www.researchgate.net/profile/Markus_Reuss/publication/317011600_Seasonal_

Metal Alloys and Chemical Hydrides seem to be up there in 6 kWh/l zone though https://www.researchgate.net/profile/Markus_Reuss/publication/317011600_Seasonal_storage_and_alternative_carriers_A_flexible_hydrogen_supply_chain_model/links/591e8a0645851540595da11d/Seasonal-storage-and-alternative-carriers-A-flexible-hydrogen-supply-chain-model.pdf?origin=publication_detail https://www.researchgate.net/profile/Markus_Reuss/publicatio... Not enough room? You can build a floating solar panel, that just ships over the ocean, generating charged LOHC which then is collected by a automated ship. PS: If you look at the references of this paper you can see the BMW research that is referenced.