See, I imagine that since you can't cook underwater (though you can freeze stuff, if you dive to sufficient depths), Ushijima is 100% FASCINATED by human foods. Especially things with textures that wouldn't work underwater, like meringue.

So basically Tendou has this daily ritual where he stops by convenience stores o bakeries and grabs something for Ushijima to try. And on special days, he gets takeout from a restaurant for Ushijima to try. And it's their little ritual and Tendou just really enjoys watching Ushijima experience all these new things.