Saline is sodium chloride in water, if it has glucose or any other additives it is no longer plain saline. You can get D5NS which is Dextrose 5% in Normal Saline (0.9% sodium chloride). Lots of different IV solutions out there which are indicated or contraindicated for some conditions/situations.

Sea/ocean water is significantly hypertonic, averaging 3.5%. Hypertonic saline has been used for treatment of patients in shock due to significant blood loss but I would not use it for dehydration and would look for a source of plain water. Hypertonic saline causes a shift of fluid from cellular and intracellular space to the circulatory system which is not desirable in dehydrated patients.

A way to get fluids into patients that can not tolerate fluids by mouth is rectally. Yes this is not an ideal means but is an option. Ideally you would have some delivery device such as tubing and a reservoir. The rectum is highly vascular and allows for rather quick absorption of medications and fluid. Not quite as fast as IV but faster than some other routes. Here is a short article on the use and success of rectal fluids, also referred to as proctoclysis: http://www.ncbi.nlm.nih.gov/pubmed/9601155

ETA: I would much quicker to put fluid that wasnt sterilized and purified in a lab type environment in the rectum vs. a vein. The rectum is more accustomed to dealing with bacteria moving through it than the circulatory system.