[Python] Geopandas checking whether point is inside polygon

I have ocean geopandas which contains 1 multipolygon (source: naturalearthdata.com)

I also have another dataframe that contains at lot of longitude and latitude information

I want to add a new column that will be True if the Point is in the ocean (inside the multipolygon)

zipfile = "ne_10m_ocean/ne_10m_ocean.shp"
ocean_gpd = geopandas.read_file(zipfile)

df = pd.DataFrame({
'lon': [120.0,120.1,120.2,120.3,120.4],
'lat': [10.0,10.1,10.2,10.3,10.4]
})

for index, row in df.iterrows():
df.loc[index,'is_ocean'] = ocean_gpd.contains(Point(x['lon'],x['lat'])


but it is too slow, I tried to used lambda function like this

df = df.assign(is_ocean = lambda x: ocean_gpd.contains(Point(x['lon'],x['lat']))


but failed, the error is cannot convert the series to <class 'float'>

Is anyone know how to do better individual point checking like this in geopandas?

Note: I just realized that for polygon data I used 10m one (more detailed polygon), if I uses 110m it a lot better, but in the future maybe I need to use 10m

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