In my first science/ cake crossover post here is a turbulent ocean cake made for a PhD viva (spot the langmuir turbulence…).
The main cakey thing here is that royal icing makes a good ocean. I added 3 tsps of glycerine to 500g royal icing sugar, could have got away with 2. Colour the icing to a bluey green base then add some blue and green colouring and mix a small amount to spread the colour without mixing it in. Save a little white icing for the flicks of foam on the waves. The waves can be made easily with a flat knife.
Here you don’t need a marzipan base for the royal icing if the cake is sponge (sometimes the colour can bleed through from cake to icing, especially if fruit cake). One thing to be careful of though is cake crumbs getting through to the icing- add a thin base layer of icing gently and then build up the thicker parts for the waves.