The fate of the red blood cell is determined by the environment surrounding it.
All animal cells exist in a slightly salty environment - there are constantly a variety of solutes, including the familiar salt sodium chloride, moving in and out of cells and existing outside of them. These are necessary for the cell to maintain its metabolic processes.
If the cell is placed in a different environment, the behavior of the cell will depend upon how closely that environment matches, or differs, from the environment that it's accustomed to. Water can flow freely in and out of the cell, and will always travel in the direction of the higher concentration of solute. Since the cell already has some solutes in it, that leaves three possibilities; either the environment has more solute, equal solute, or less solute, compared to the cell's natural environment.
The cell's natural environment is the inside of the body, so we would expect this to be isotonic, leading to no net flow of water in or out of the cell. It would remain the same size.
In the ocean, the environment is extremely salty compared to the inside of the cell. We would expect water to flow out of the cell, toward the higher concentration of solute. This would cause the cell to lose water, and therefore it would shrivel and shrink. This environment would be hypertonic, and the cell would undergo plasmolysis.
The example of a public pool might be slightly mistaken - while the pool would most likely be filled with fresh water, public pools are often chlorinated, which would mean that there is some concentration of solute in the pool. Whether it is higher than the cell's environment and contents or not, I can't say, but I'll assume it's less. Either way, pure water would be a better example.
In pure water, the environment around the cell has fewer solutes than the cell itself. This would cause water to flow into the cell, until it expands and bursts. The environment is hypotonic, and the cell would undergo cytolysis.