Don’t ask for your tap water with no ice or lemon. It sets a bad tone with your server that you will be high maintenance even if you won’t be. If you must, wait until you’ve ordered a drink or—if you’re not drinking—talk about the menu with your server first then ask for some lemon wedges on the side or to make the water ice-less if he wouldn’t mind. If you’ve ordered bottled water, you can do whatever you want because it isn’t free.
Never make false claims of a food allergy to justify altering the ingredients of a dish. If garlic gives you gas, for example, that should not be construed as an allergy. Most servers can detect you’re lying when you spontaneously become gluten intolerant after we refuse to make a substitution, so come correct. Staff in restaurants take food allergies very seriously, so should you.