**2. Materials and Methods**

#### *2.1. Study Site and Data*

### 2.1.1. Study Site

The study site is located in the northwest of the island of Mayotte (latitude: 12.63◦−12.68◦S, longitude: 45.10◦−45.15◦E), a French overseas territory located in the southwest Indian Ocean (Figure 1). This site is partially sheltered from wind and wave influence, factors known to affect the quality of SDB estimates. Mayotte coasts offer a wide variety of marine fauna and flora strongly affected by global climate and local anthropogenic changes, requiring environmental monitoring [1,3]. This region was also selected due to the availability of high-resolution airborne bathymetric LiDAR and multibeam echosounder data necessary for validation.

**Figure 1.** Map of the ground tracks of ICESat-2 over Mayotte collected on 14 May 2020. The satellite multispectral imagery was acquired by Pleiades-1A on 25 May 2020. The red square identifies the study area.

Water clarity is a key parameter in SDB estimation. Clarity is related to light ray penetration in the water column, thus impacting the quality and quantity of the available bathymetric soundings [27–31]. This variable can be estimated using a diffuse attenuation coefficient of 490 nm measured at 4 km resolution by the moderate-resolution imaging spectroradiometer (MODIS-Aqua, publicly accessible from https://oceancolor.gsfc.nasa. gov/l3/, last accessed: 28 December 2021) [32]. A diffuse attenuation coefficient value of 0.0615 m−<sup>1</sup> was obtained for the month of May 2020 for the study site, indicating a very clear water type. Previous studies using ICESat-2 for bathymetric estimations had a diffuse attenuation coefficient ranging from 0.032 m−<sup>1</sup> for the Virgin Island to 0.123 m−<sup>1</sup> for the Bahamas, both known to be areas with very clear water [21,33].
