As I understand it, one common way to solve this is to have a ...Base package (something like ClimateDatasetBase.jl) where you declare
function getLandSea end
and then have
import ClimateDatasetBase: getLandSea
function getLandSea(...)
...
end
in both your packages, to make them actually be methods of the same function.
Whether this is worth doing of course depends on how many such functions you have, but such a base package also (1) lets you define more of a common interface between the packages, (2) is a place to define Abstract... types for the child packages to subtype, and (3) can make it easier for others writing a climate dataset-handling package to integrate better with your existing ones.