For the decoration, you can use this guide on how to make a palm tree and make a few on the sidelines, and things like that. If you want to make a towel, you can use barrier, and decorate it or just use… IDK. You can make some things like the very small ground breaking plant, and alien plants, for grass. To make the water, I suggest using different colours, and make it look like the different layers in the zones of water. Adding shells, on the beach, different shapes, sizes and colours. Something I personally like to do ( I don’t know it anyone else does, ) but I like to put coral on above the water to give it ocean vibes.
If you want a plot...
For the plot, you can make ideal: One day, two people were going for a swim in the ocean, to take time away from there house. When, one day, there is a giant sea creature ( Kraken maybe 
) comes out of the ocean, but they have to defeat this Kraken in order to save the people around them. 
Do you need anymore? I can come up with some! 