The sims can swim in those ponds you place from debug. Just place a diving platform near it (think you need to use MOO cheat) and they will dive into the pond and swim around and then teleport themselves out of the pond. Use the largest pond size for this. You can even put pond plants and effects on them and you can fish there too. I have such a pond in my game and it works beautifully.
For pools, would be nice to have one of those trims that look like brick, it's usually used for man made ponds, but a natural looking pond trim would be great too if they also make it match the colour of the neighboring terrain.