Word list sigils paired with Enum.zip or Enum.zip_with could be nice here.

[_columns, _units | spectral_data] =
  body
  |> String.split("\n", trim: true)
  |> Enum.map(&String.split(&1, " ", trim: true))
  ...

# word list sigil
column_headers = ~w[
  year month day hour minute
  wave_height swell_height swell_period
  wind_wave_height wind_wave_period
  swell_direction wind_wave_direction
  steepness average_period mean_wave_direction
]a # type modifier for atoms

first_row_map =
  column_headers
  |> Enum.zip(hd(spectral_data))
  |> Enum.into(%{})
# or
first_row_map =
  spectral_data
  |> hd() # or `Enum.at(0)`
  |> then(fn first_row -> Enum.zip(column_headers, first_row) end)
  |> Enum.into(%{})
# or even
first_row_map =
  spectral_data
  |> hd() # or `Enum.at(0)`
  |> Enum.zip_with(column_headers, fn [c, h] -> {h, c} end)
  |> Enum.into(%{})