Hydrographic survey is the best way to get a reliable data. But there are some techniques to get bathymetery using remote sensing.

Also, you can download bathymetery data from the below link.

https://www.gebco.net/data_and_products/gridded_bathymetry_data/

There are different ways to get a bathymetric data with a wide range of acurracy and intervals.

off course the most accurate with desired interval data is provided by field measurmnet bathymetry survey and such kind of data could be provided by coastal authorities at research's gografical zone area or from data collection duirng a project at the same area .

the other and easier ways are to get bed levels data from admirality chrtsr, you can easily download their apps and searching by area to get the chart number, and/or to get the data through free websites such as www.gebco.net

Besides conventional vessel based bathymetric surveys, there is another possibility to extract depth information using satellite derived bathymetry approach.

U can here - NOAA (National Oceanic and Atmospheric Administration) is the primary source of bathymetric data for the world's oceans. See their Bathymetry and Global Relief website