the mooncup should be quite low down - if its uncomfortable, you should snip the stem off (or v. short) As long as you put in a clean tampon/mooncup, and change/empty it after you get out of the pool, you should be fine.
Disposable soft cup things - more comfortable than a mooncup and great if you normally used pads etc. Now good if you have a coil though (speaking from experience of "Oh, it will be fiiiiiiiiiiine" then losing the string )
I have oodles here from where I bought them originally to sell at bumfluff - more than happy to sell some
Most people wash and reuse a few times even though they are disposable, so it really depends on how often you're likely to use them, how often you think you'd reuse etc. Makes sense to me to buy just 6 to start with though incase you don't like them