It's a bit of a generalization, but usually the Water Snakes bite is a bit worse as they are larger and have larger teeth. The bite from a Garter doesn't really hurt, it just makes you jump! If they do break the skin it's just like little pin pricks. Water Snakes bite a bit deeper and can cause a little bleeding. The bite from a very large Water Snake can be quite unpleasant. (Trust me!)
If you were to get bitten, you would just need to treat the bite as you would any cut or abrasion, clean it well.
