Preheat your oven to 190°C (Gas mark 5/375°F). Grease and line 3 x 20cm round cake tins (shallow sandwich tins are ideal).
Place the butter and caster sugar in a large bowl and whisk together until well combined and fluffy.
Add the six eggs, two at a time, whisking after you add each pair.
Once the eggs have all been whisked in, add the flour, baking powder, milk and vanilla. Whisk again until fully combined.
Divide the mixture between the 3 tins, gently smoothing the tops down with a spatula.
Bake in the pre-heated oven for around 20 minutes, until the cakes are golden and springy to the touch.
Turn the cakes out onto cooling racks and leave to cool completely before icing.
Whilst the cakes are cooling, prepare the buttercream icing.
Whisk the butter until soft and fluffy. Add the icing sugar, a little at a time, whisking as you go until completely combined. Add the vanilla extract and whisk again until combined.
Remove approx 300g of the buttercream icing and pop it into a separate bowl - this will be used to fill the cake.
Remove another 300g of the icing - this will be used for the crumb coating.
Colour two-thirds of the remaining icing blue, and the remaining third green.
When ready to assemble, secure one of the cakes onto a cake board with a little of the buttercream icing.
Spread over half of the buttercream icing you set aside for filling, and top with a couple of spoonfuls of raspberry jam (optional). Add a second cake and repeat the buttercream and jam filling, then top with the third and final cake.
Using a palate knife, spread the buttercream that you set aside for the crumb coating all over the cake. If you have a cake turntable, this will really help you to get an even coat. Spinning the cake slowly, use a large cake scraper to even out the sides.
Once the cake is covered in the crumb coating, dab small amounts of green icing all over the cake. Continue with the blue icing, until the whole cake is covered in dabs of blue and green buttercream. Using the cake scraper, smooth the icing until roughly even and blended together. Bear in mind you're going for an ocean effect so it doesn't have to be perfectly smooth.
Decorate the cake with the plastic sea creatures and top with the pirate ship decoration. I pushed my ocean creatures into the buttercream around the base of the cake, and added an octopus to the top.