I would recommend creating a GitHub Organization (maybe JuliaHydroModels) to host all these packages, instead of tying them to your personal user account. That way, you can have additional co-maintainers, and it’s not such a problem if you ever step back from the project.
Please make sure that every package has at least a README that explains the purpose of the package and how it relates to the main HydroModels.jl.
Assuming each package is ready for registrations, you may want to consider manually preparing a Pull Request (with the help of LocalRegistry package) that registers all of them in one go (actually, don’t: Gunnar’s suggestion is a far better approach) . Otherwise, you have to deal with registering them in the order of their dependency graph, with a three-day period for each package.