the same as suggested here: Radiometric & Geometric Correction Workflow - #35 by ABraun
No Terrain Correction is required (or possible) over water so you use the Ellipsoid correction.
the Wave mode has a higher spatial resolution but is not available for most parts of the earth: https://sentinel.esa.int/web/sentinel/user-guides/sentinel-1-sar/acquisition-modes
Is there a reason why you want to stick to SLC data? Both SUMO and SNAP’s ship detection make use of GRD and you can skip many steps to create the ground range data.