Right before you fight the Clam boss turned mermaid, you'll come to a room with shell girls and venuses on both sides of a corridor that goes straight up. You can use the water filter to fill up the place but it's easier to just wall jump it. On the left side of this corridor will be a hidden entrance, follow that all the way to it's end and you'll find the golden hammer.
Be warn though, the place is overran with the 2 enemies and you can easily get knocked out of girl form and raped to near death if you haven't been finding the health upgrades through out the game.
P.S. There's no way I can try and do primarily male he moves too damn slow, hell I can't even stand the 1st girls running speed and usually switch to the 2nd girl since she's fast and doesn't bounce all over the place like 3rd girl. If the game itself was slow-paced it'll be fine. But he's way too slow for the speed of this game.
