Ending bonus:
Beat the game and start a new one. Select the "Leave Tomorrow" option at the hotel or pool menu to skip to the ending. You'll also get two new songs.

Start with more money:
This requires that you have DOA 3. Play Survival mode in DOA 3 with a character that also appears in Xtreme Beach Volleyball. Save your game and start Xtreme Beach Volleyball with the same character. The more wins you have in Survival mode, the more money you will start with in Xtreme Beach Volleyball.

Bathing Beauty Tip:
The girls on the island tan quite nicely. They only tan when playing volleyball on a sunny beach, however. They will not tan when playing in the "jungle." If you want your beauty to tan faster, buy some suntan lotion from the Accessory Shop and apply it before you play a game (do not put it on when you're back at the hotel the night before, or its useless). If for some reason you don't want your babe to tan, apply some sun block lotion (also purchased from the Accessory shop) and apply it at the start of the day (it lasts all day).

See-through suits:
Play as a girl with a transparent suit. Equip a transparent visor as well. Go to the pool and allow your girl to lay on the chair until the menu disappears. Move the camera so that you are looking at the girl's body through the visor. Stars censor out the risqué parts.
 

Thanks to Revolution readers Cornholio, Scott "GroZZleR" McCallum, Malcom Eddie, and Geoff Custer!