For pure recreation, the beach definitely! I like letting the sun warm you up as you lay out tanning, and then running into the water to cool off. Or taking long walks along the beach collecting shells, etc.
However, for exercise, I like pools better, esp. ones made specifically for lap swimming. I was a competitive swimmer since before I was 8-years old, so I really like to lap-swim for my aerobic exercising. They usually keep the water cooler (so that you don't over-heat when you're doing laps), the size is regulated (usually either 25-yards like most high-school pools or about doubled out to 50-meters for Olympic-style pools), there aren't huge waves, and there usually are some type of lane dividers to help keep people separated.
What I really dislike are pools that are meant for exercising yet are missing one or more of the stipulations that I mentioned above. For example, there's a health club in town that has a pool that is maybe 20 yards in length (being generous) and is more amoebiod in shape. Also, the local YMCA has a regular 25-yard "adult" pool (in addition to a secondary, general access pool that they used for recreational swimming as well as swim lessons) that they unfortunately keep heated way too high -- the reason being that their elderly swimmers complained about it being too cold when they had originally set it to a more optimal temperature for more rigorous lap swimming.