Pools are big and wonderful, but they can easily get in the way and disturb level flow. These are the easiest objects to abuse. The first park I tried was horrendous for this reason. I put a group of penpendicular rails perpendicular. It was a mess.
Rails are best when used in conjunction with other lips or looping the backside of an object.
Speaking of rails, if you are attempting to place them for transferring between one another, be sure you use regular rails and offset rails. Otherwise, the gaps will be too big. If you are interested in placing a halfpipe on your map, keep it out of the center. A halfpipe takes up too much space in the middle of a board. Keep things simple.