When I was 5, I too had the swimming pool cake as my birthday cake, but as I was a Picky Young Lady, I didn't want jelly in the middle. Blue icing sufficed.
When I turned 25, I asked Mum to do the cake again, seeing it was such a hit the first time round. Now that I look closely, it was more of a desert island than a swimming pool!
For those that DID like jelly, we also had frog in a pond.