Fiberglass is often used to resurface concrete pools. Theoretically, fiberglass is superior to plaster as it is waterproof. This means it can withstand stains, has a smooth texture, and comes in many different colors.
Paint: While it is not a long-term option, paint is an economical choice. The Band-Aid will save you time and allow you to put aside money to remodel your pool in the near future.
If you want to save money on tiling, you may consider sticking to accent tiling. You can finish the underwater portions of your pool with a cheaper material, such as paint or plaster, and lay tile down around the edges for a striking statement.