1. Add in more info_player_deathmatch entities to your map.
2. You can put a trigger_hurt a unit above the water and set the damage to a negative amount, which will heal the player instead of hurting. Trigger_hurt is a brush-based entity, like func_water, so create a brush textured with null and turn it into a trigger_hurt.
3. Not off the top of my head, but there are
places that do textures. You'd have to turn them into Half-Life .wad files yourself, which someone else may have to help out with because I don't remember how. Perhaps someone would like to
write a tutorial on the subject?