Here is a modification that contains a map mod. This will add an island type which can be used for deathmatches if you have a multiplayer client such as SA-MP.
NOTE: Please backup your entire Data folder first before you install this mod. I am not liable for your actions.
Step 1: Add deathmatch.IPL to GTA San Andreas/Data/Maps/* ADD HERE*.
Step 2: Add and replace gta.dat in GTA San Andreas/Data/*ADD AN REPLACE HERE*
Step 3: Test and play. An Have Fun!
Its across from the San Fierro Airport Control Station on the right side of the run way.