There is no known cheat code that spawns a boat directly in the PSP version of GTA: Vice City Stories.
Head to any major body of water and check the docks—boats spawn regularly. The Speeder and Squalo are the most common. For the Marquis (yacht), check the Bayside Marina docks.
Note: Cheat availability varies by region (ULUS/ULES) and firmware. Always verify codes for your exact version.