default url is https://upwell.pfeg.noaa.gov/erddap/
You can set a default using an environment variable so you
don't have to pass anything to the URL parameter in your
function calls.
In your .Renviron file or similar set a URL for the environment
variable RERDDAP_DEFAULT_URL
, like
RERDDAP_DEFAULT_URL=https://upwell.pfeg.noaa.gov/erddap/
It's important that you include a trailing slash in your URL