Well here is one way to keep Your Blue Mind Turned On at home! :-) Check out this video on how to make a table that looks like the ocean. It is a good first try.
I think some white/beige concrete might be better...gray looks more like the lake but maybe you rather that. I think also you can play with trying to get the deep side darker blue. Maybe a divider with darker blue then take the divider out & do the lighter blue. That may just be two colors only and you might want a third for a bit of gradual darkening? Something to think about & try.
I think you could also put some small shells or shell pieces on the shallow side before you put the epoxy in and hey, you could make it fun with some plastic colorful fish or a SHARK! HA! You know those kids ones! :-)
Here is the full page from This Old House with directions.