btw while taking the enoden is scenic it's adding like an additional half-hour to the trip and doubling the cost to get to the Katase/Enoshima area from Shinjuku. also the monorail is much cooler.
that being said if you're in the Tokyo area in the summer, the beaches there are a little over an hour away and make for a great day trip. the beach is public and there's places to shower and change in the area.