"Day of water splashing" in Japanese

And:

Day of Ocean splashing

Do you mean a day of you/someone playing with/splashing around in ocean? or do you mean simply the waves coming in and going out all the time? And what you are looking for is something poetic, or matter-of-fact expression/description?

it does sound unusual en English! I have heard/ read "we spent hours splashing around in the Ocean (or lake, pool etc.)"
this reminds me the game played in lower grades. the teacher would whisper a sentence in one child ear who would whisper it to another one and so on. at the end the first and the last child would write down the sentence they heard. usually not a single word was the same..