It honestly does sound kind of familiar but it can't think of any particular title.
This does have a Ferris wheel sex scene: http://www.mangago.me/read-manga/only_you_tohjoh_asami/tr/94054/page/
Thought it doesn't have the water scene you described.
I recommend just asking in a random yaoi comments section. I've done that before and someone always knows ٩(๑❛ᴗ❛๑)۶
It's bugging me because I remember what it had and it kinda looked like it came from the artist who did haru wo daiteita.
I wish I could help you! I've been there before
Nice collection. Do you know the name of a manga it's a collection of one shots and one of them has 2 guys having sex on a ferris wheel and falling into water.