Your story was wonderful I hope I can read the rest of the book I hope you can come up with the best of the ideas to complete the story and You will become a great writer I wish you the best of luck

I NEED SMUT! GIVE ME, GIVE ME, GIVE ME!

This is a good start, I am eager to see where this goes.
I totally agree with AwkardOtaku16's comments about format, etc. I don't think there is anything to add to that. Your story so far is well written and looks like it will be a lot of fun.
As for where you might take it. My thoughts instantly went to Akihiko's attire for the beach. I was thinking knowing him, it'd be dark glasses, short sleeved shirt and trousers sitting under a parasol with his laptop. But, then, speedos jumped to mind and the comical value of them. I'm not sure in which direction to point you in, there are so many variants, also, I'm a little reluctant to know where a story is heading before I get there. Still, I am excited to see where this goes.
Thank you for sharing your work with us, I appreciate it. I look forward to the next chapter. :)

I love how you have Usami pout..When reading him sit on the counter I felt the eruption of mount Misaki brewing. Very fun so far. My only demand is Usami suffering sunburn at the end haha. Very sweet and heart warming...Please continue to write your writing style is adorable.
The characters are very in character as well...Which is appreciated. ;-)

The title of this made me laugh because there seems to be this unspoken rule in anime that all the characters have to visit the beach in one episode XD Maybe some of the other couples could make cameos in the upcoming chapters?
Your writing style is descriptive without an overload of detail, which I liked. Although, I notice you don't start a new paragraph when someone starts speaking, which is something you should always do. For example, instead of "Misaki began to sputter and shouted "Now is not the time you pervy rabbit!" Usagi just pouted and whined "But I need my fill of Misaki!" "Well tough luck!"", it should read:
'Misaki began to sputter and shouted, "Now is not the time, you pervy rabbit!"
Usagi just pouted and whined. "But I need my fill of Misaki."
"Well, tough luck!"'
This makes the story a lot easier to read because it's more clear who's speaking, and it also breaks the story down; the main body of this chapter is one long paragraph, so if you start a new line whenever someone starts speaking it'll look better as well. :)
However, your spelling and grammar is good and both Misaki and Usagi-san are very in-character. And I like the overall idea, too- it's cute, funny and open to lots of different scenarios. I'm looking forward to seeing what they get up to at the beach... So, good job with the first chapter!
As for ideas, I think there's tons of cute little moments that could happen in a situation like a beach trip that you could incorporate, but I also think it would be funny if Misaki lost Usagi-san or something. If you haven't seen it, there's a chapter in the manga where they go to a festival, and Misaki gets separated from Usagi-san and freaks out for a couple of minutes. Maybe you could do something like that, like, Misaki thinks Usagi-san has drowned or something? Anyway, you're the author, so it's up to you!
Sorry for the really long review DX Still, I hope it was helpful. Good luck with the story and update soon! :)