Following the news of Agatsuma Ent. becoming defunct, the worse has happened a little too soon:
The Umihara Kawase games have been delisted from Steam.
It is unknown if the games will return under a new publisher (someone go get Night Dive Studios on the line), but the only other way of getting the games now would be through traders who have kept the games in their inventories.
EDIT2: The 3DS ver. of the game was published by Natsume and as such that ver. of the game won't be delisted.