Help - platform over pool

Ok so your main issue here is you are trying to build a platform over an existing pool, that doesn't work. You have to shape the pool by hand to form the shape you want and form a "platform" from the empty space between. If you click the pool, you should be able to drag the platform towards the archway if that helps.

To remove the pool trim from the middle part, you have to first remove all the trim from the entire pool, then hold shift and click each pool wall you want the trim on individually.

As far as platforms directly over pools goes, that's not an option. A walkway must be at least one story above the pool to go over it.