Near the edge of the public beach in Manuel Antonio is the exit to the national park. If you leave the park, you can get a stamp to let you back in (funny how the stamp was placed on my forearm and the humidity quickly made it a black inkblot-looking stamp instead).
During the early afternoon hours, when the high tide is in, there are boats that take people from the exit to the public beach. There is really no way around the rising swell of water unless you wait in the park (high tide is highest from 11 am to 2 pm) or swim.
Swimming is not recommended. Why? There are two crocodiles there. We didn't see one but we didn't doubt the people who told us about the crocs (one of which was our tour guide).
I took this picture before I knew of the crocs, when I wasn't even sure were used when I took it. It was about 4 pm local time and the tide was way down. The boats seemed more decoration than anything.
Made for a nice picture.