Yes, sea snakes can.
Chlorine that is used up or spent usually causes itchy skin and/or a rash. To combat this add more chlorine to burn off the used chlorine, this will also reduce the chlorine smell. Well, the above answer is partially right. What is causing your skin to itch is improper pH balance. It is either too high or too low. It should be in the range of 7.4 ppm. Adding more chlorine will burn off the "chloramines" not the "used chlorine". Once the chlorine is used there is no more chlorine. Adding more chlorine will reduce the smell of chlorine or in proper terms the smell you are getting of chlorine is called chloramines.
Yes the low concentration of salt in a salt water pool would not ham fiberglass any more then much stronger concentrations of salt in sea water water harms the hull of a fiberglass boat.
You don't use Muriatic acid to clean pools. It is used according to manufacturer's directions to balance the pH of your water. Acid v Alkalinity. Too high a pH, add acid. To low, add a pH increaser. 7.0-7.2 is about right. Now, your pool maintenance people may suggest a muriatic wash for your pool if it is loaded with black (or other) algae. However, that should be done only by trained, experienced professionals as we are talking about ACID. This acid in the water, administered properly will not have adverse effects; this I know because I once let my large in-ground pool go bad for a Southern California winter, and it became a breeding ground for mosquitos. I dumped in several gallons of acid to kill them and it didn't hurt them a bit! I hired a pool man and he said that the killer substance is the chlorine! (I got the problem cleared up before the little swimming buggers hatched into flying buggers!) If you are talking about acid washing a pool it is best to hire a professional to do the job. If an acid wash is suggested to remove algae, call someone else until they tell you a chlorine wash is the best way to remove algae stains. Sometimes both are needed to remove algae and metal stains. Try to maintain a pH of 7.2-7.4. If you are going to acid wash your pool find someone who has done a couple hundred of them as I have replastered many pools because an idiot did an acid wash improperly.