I Tried The Ocean Modifier to make an ocean with depth like the one on the image:

but I Get A Flat Ocean. Please I Need Help
I Tried The Ocean Modifier to make an ocean with depth like the one on the image:

but I Get A Flat Ocean. Please I Need Help
If you don't care about the bottom of it being deformed like the top, you can simple add a solidify modifier (with offset of -1) to achieve this effect. Otherwise, this answer is suitable.
Just fill a water domain up to where you want. Make sure the material of the water is set to glass and and IOR of 1.333. With the nodes you can tweak the look of the water even more. And don't forget to light it.