To spawn the Boat, go to the location where you've installed GTA1, then open the GTADATA folder, find a file called MISSION.INI. Open it in Notepad and find the line:
293 1 (106,119,4) PARKED 31 0
The second number from the right determines car model. The default "31" spawns a Bulldog, changing it to "13" will spawn a Boat. Save changes, run GTA1 (Liberty City, chapter 1) and that's it. You can also change coordinates (in brackets) to spawn the Boat next to some water - for example, in this video the Boat is spawned at 194,59,3. (less info)