Handel wrote probably the most famous "Water Music": a series of suites for King George I who wanted a concert on the Thames.
This list is of much lesser-known works. Several are by living composers. Not all of them are as easy on the ears as Handel, but variety is the spice of life.
Schubert - Impromptu No. 2 in E-flat major (*This piece isn't specifically about water, but listening to it, you could easily imagine a quickly moving river or fountain.)
Moritz Moszkowski - 6 Morceaux, Sur l'eau (On the Water)
Pantcho Vladigerov - Water Colours
Luecht - Columbia River Suite
Hubert Parry - A Garland of Shakespearean and Other Old-Fashioned Songs; No. 5, A Sea Dirge
Juhan Puhm - Of Sky and Water (Transformation)
Hakon Børresen - Symphony No. 2 "The Ocean"
Valentino Bastianelli - Storm on the Ocean
Heise / Lange-Muller - 3 Songs, Akande (Water-Lily)
reorder
Princes love hearing symphonies. Jesters hate humming melodies vehemently.